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ZOOLOGY. 

ANIMAL  STUDIES.  A  one-book  course  in 
Zoology  for  secondary  schools.  By  DAVID 
STARR  JORDAN,  President  of  Leland  Stanford 
Jr.  University ;  VERNON  L.  KELLOGG,  M.S., 
Professor  of  Entomology,  Leland  Stanford  Jr. 
University;  and  HAROLD  HEATH,  Professor 
of  Zoology,  Leland  Stanford  Jr.  University. 
Cloth,  $1.25  net. 

ANIMAL  LIFE.  A  First  Book  of  Zoology. 
By  DAVID  STARR  JORDAN  and  VERNON  L. 
KELLOGG.  Cloth,  $1.20  net. 

ANIMAL  FORMS.  An  Elementary  Text-Book 
of  Zoology.  By  DAVID  STARR  JORDAN  and 
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foregoing  in  one  volume.)  Cloth,  $1.80  net. 

TEACHER'S  MANUALS. 

ANIMAL  STRUCTURES.  A  Laboratory 
Manual  of  Zoology.  By  D.  S.  JORDAN  and 
GEORGE  C.  PRICE,  Associate  Professor  of  Zo- 
ology, Leland  Stanford  Jr.  University.  Limp 
cloth,  50  cents  net. 


D.  APPLETON  AND  COMPANY,  NEW  YORK. 


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TWENTIETH   CENTURY  TEXT-BOOKS 


ANIMAL    LIFE 

A  FIRST  BOOK  OF  ZOOLOGY 


BY 

DAVID  STARR  JORDAN,  PH.  D.,  LL.  D. 

\N 

PRESIDENT   OF   LELAND    STANFORD   JUNIOR    UNIVERSITY 
AND 

VERNON  L.   KELLOGG,  M.S. 

PROFESSOR   IN    LELAND    STANFORD   JUNIOR   UNIVERSITY 


NEW    YORK 
D.    APPLETON    AND    COMPANY 

1912 


COPYRIGHT,  1900 
B*  D.   APPLETON  AND  COMPANY 


BIOLOGY 

LIBRARY 

G 


PREFACE 


THE  authors  present  this  book  as  an  elementary  ac- 
count of  animal  ecology — that  is,  of  the  relations  of  ani- 
mals to  their  surroundings  and  their  responsive  adaptation 
to  these  surroundings.  The  book  takes  the  observer's  point 
of  view,  who  is  especially  concerned  with  the  reasons  for 
the  varied  structure  and  habits  of  animals.  To  understand 
how  naturally  and  inevitably  all  animal  form,  habit,  and 
life  are  adapted  to  the  varied  circumstances  and  conditions 
of  animal  existence  should  be  the  motive  of  the  beginner  in 
this  fascinating  study.  The  greatest  facts  of  life,  except 
that  of  life  itself,  are  seen  in  the  marvelously  perfect  meth- 
ods which  Nature  has  adopted  in  the  structure  and  habits 
of  animals.  The  keen  observation  of  a  fact  should  lead 
the  student  to  inquire  into  the  significance  of  that  fact. 
The  veriest  beginner  can  be,  and  ought  to  be,  an  independ- 
ent observer  and  thinker.  In  the  study  of  zoology  that 
phase  which  treats  of  the  why  and  how  of  animal  form  and 
habit  not  only  absorbs  the  attention  of  the  most  advanced 
modern  scholars  of  biology,  but  should  also  appeal  most 
strongly  to  the  beginner.  The  beginner  and  the  most 
enlightened  thinker  in  zoology  should  each  have  the  same 
point  of  view.  With  this  belief  in  mind  the  authors  have 
tried  to  put  into  simple  form  the  principal  facts  and 
approved  hypotheses  upon  which  the  modern  conceptions 
of  animal  life  are  based. 

It  is  unnecessary  to  say  that  this  book  depends  for  its 


vi  ANIMAL   LIFE 

• 

best  use  on  a  basis  of  personal  observational  work  by  the 
student  in  laboratory  and  field.  Without  independent 
personal  work  of  the  student  little  can  be  learned  about 
animals  and  their  life  that  will  remain  fixed.  But  present- 
day  teachers  of  biology  are  too  well  informed  to  make  a 
discussion  of  the  methods  of  their  work  necessary  here. 
As  a  matter  of  fact,  the  methods  of  the  teacher  depend  so 
absolutely  on  his  training  and  individual  initiative  that  it 
is  not  worth  while  for  the  authors  to  point  out  the  place 
of  this  book  in  elementary  zoological  teaching.  That  the 
phase  of  study  it  attempts  to  represent  should  have  a  place 
in  such  teaching  is,  of  course,  their  firm  belief. 

The  obligations  of  the  authors  for  the  use  of  certain 
illustrations  are  acknowledged  in  proper  place.  Where  no 
credit  is  otherwise  given,  the  drawings  have  been  made  by 
Miss  Mary  H.  Wellman  or  by  Mr.  James  Carter  Beard,  and 
the  photographs  have  been  made  by  the  authors  or  under 
their  direction. 

DAVID  STARR  JORDAN, 
VERNON  LYMAN  KELLOGG. 


NOTE.— After  the  pages  of  the  book  were  cast,  it  was  thought  that 
a  transposition  of  Chapters  III  and  IV  would  present  a  more  logical 
arrangement,  and  teachers  are  advised  to  omit  in  their  study  scheme 
Chapter  III  until  Chapter  IV  is  completed.  D.  S.  J. 

V.  L.  K. 


CONTENTS 


CHAPTER  PAGE 

I. — THE   LIFE  OF   THE   SIMPLEST   ANIMALS    .  .  .  .  •:     .  .  1 

The  simplest  animals,  or  Protozoa,  1.— The  animal  cell,  2. — 
What  the  primitive  cell  can  do,  5. — Amoeba,  5. — Paramoecium,  9. 
— Vorticella,  12. — Marine  Protozoa,  15.— Globigerinae  and  Radio- 
laria,  16. — Antiquity  of  the  Protozoa,  20. — The  primitive  form, 
20. — The  primitive  but  successful  life,  21. 

II. — THE   LIFE   OF   THE   SLIGHTLY   COMPLEX   ANIMALS    .  .  .24 

Colonial  Protozoa,  24. — Gonium,  25. — Pandorina,  26. — Eudo- 
rina,  27. — Volvox,  28. — Steps  toward  complexity,  30. — Individual 
or  colony,  31. — Sponges,  32. — Polycs,  corals,  and  jelly-fishes,  37. 
— Hydra,  37. — Differentiation  of  tne  body  cells,  41. — Medusje.or 
jelly-fishes,  41.— Corals,  43.— Colonial  jelly-fishes,  45.— Increase 
in  the  degree  of  complexity,  48. 

III. — THE   MULTIPLICATION  OF  ANIMALS   AND   SEX  .  .  .50 

All  life  from  life,  50. — Spontaneous  generation,  51. — The 
simplest  method  of  multiplication,  53. — Slightly  complex  methods 
of  multiplication,  54. — Differentiation  of  the  reproductive  cells,  55. 
— Sex,  or  male  and  female,  57. — The  object  of  sex,  57. — Sex  di- 
morphism, 58. — The  number  of  young,  61.  , 

IV.— -FUNCTION  AND-  STRUCTURE .63 

Organs  and  functions,  63. — Differentiation  of  structure,  64. — 
Anatomy  and  physiology,  64. — The  animal  body  a  machine,  65. 
— The  specialization  of  organs,  66, — The  alimentary  canal,  66. — 
Stable  and  variable  characteristics  of  an  organ,  73. — Stable  and 
variable  characteristics  of  the  alimentary  canal,  73. — The  mutual 
relation  of  function  and  structure,  77. 

V. — THE  LIFE  CYCLE    .        .     •...»..      »,     .  .        .      78 

Birth,  growth  and  development,  and  death,  78. — Life  cycle  of 
simplest  animals,  78. — The  egg,  79. — Embryonic  and  post-em- 
bryonic development,  80. — Continuity  of  development,  83.— De- 
velopment after  the  gastrula  stage,  84. — Divergence  of  develop- 

vii 


Yiii  ANIMAL  LIFE 

CHAPTER  PAGE 

ment,  84. — The  laws  or  general  facts  of  development,  86. — The 
significance  of  the  facts  of  development,  89. — Metamorphosis, 
90. — Metamorphosis  among  insects,  90. — Metamorphosis  of  the 
toad,  94. — Metamorphosis  among  other  animals,  96. — Duration  of 
life,  101.— Death,  103. 

VI. — THE   PRIMARY  CONDITIONS  OF  ANIMAL  LIFE  .  •     ,     •  .      106 

Primary  conditions  and  special  conditions,  106. — Food,  106. — 
Oxygen,  107. — Temperature,  pressure,  and  other  conditions,  108. 
— DitFerence  between  animals  and  plants,  111. — Living  organic 
matter  and  inorganic  matter,  112. 

VII. — THE  CROWD  OF  ANIMALS  AND  THE  STRUGGLE  FOR  EXIST- 
ENCE       .       •       .    114 

The  crowd  of  animals,  114. — The  struggle  for  existence,  116. 
—Selection  by  Nature,  117.— Adjustment  to  surroundings  a  re- 
sult of  natural  selection,  120.— Artificial  selection,  120.— Depend- 
ence of  species  on  species,  121. 

VIII.— ADAPTATIONS .,       .133 

Origin  of  adaptations,  123.— Classification  of  adaptations,  123. 
—Adaptations  for  securing  food,  125.— Adaptations  for  self-de- 
fense, 128.— Adaptations  for  rivalry,  135.— Adaptations  for  the 
defense  of  the  young,  137.— Adaptations  concerned  with  sur- 
roundings in  life,  143. — Degree  of  structural  change  in  adapta- 
tions, 146.— Vestigial  organs,  147. 

IX. — ANIMAL  COMMUNITIES  AND  SOCIAL  LIFE       ....    149 

Man  not  the  only  social  animal,  149.— The  honey-bee,  149.— 
The  ants,  155.— Other  communal  insects,  158. — Gregariousness 
and  mutual  aid,  163.— Division  of  labor  and  basis  of  communal 
life,  168. — Advantages  of  communal  life,  170. 

X. — COMMENSALISM   AND   SYMBIOSIS 172 

Association  between  animals  of  different  species,  172. — Com- 
mensalisrn,  173. — Symbiosis,  175. 

XI. — PARASITISM  AND  DEGENERATION 179 

Relation  of  parasite  and  host,  179. — Rinds  of  parasitism,  180. 
—The  simple  structure  of  parasites,  181.— Gregarina,  182.— The 
tape- worm  and  other  flat- worms,  183. — Trichina  and  other  round- 
worms,  184.— Saccul'na,  187.— Parasitic  insects,  188.— Parasitic 
vertebrates,  193. — Degeneration  through  quiescence,  193. — De- 
generation through  other  causes,  197. — Immediate  causes  of  de- 
generation, 198. — Advantages  and  disadvantages  of  parasitism 
and  degeneration,  198. — Human  degeneration,  200. 


CONTENTS  ix 

CHAPTER  PAG* 

XII.— PROTECTIVE  RESEMBLANCES  AND  MIMICRY    .        .        .       .201 

Protective  resemblance  defined,  201. — General  protective  or 
aggressive  resemblance,  202.— Special  protective  resemblance, 
207. — Warning  colors  and  terrifying  appearances,  212. — Alluring 
coloration,  216. — Mimicry,  218. — Protective  resemblances  and 
mimicry  most  common  among  insects,  221. — No  volition  in  mim- 
icry, 222.— Color :  its  utility  and  beauty,  222. 

XIII. — THE  SPECIAL  SENSES 224 

Importance  of  the  special  senses,  224— Difficulty  of  the  study 
of  the  special  senses,  224. — Special  senses  of  the  simplest  ani- 
mals, 225.— The  sense  of  touch,  226.— The  sense  of  taste,  228.— 
The  sense  of  smell,  229.— The  sense  of  hearing,  232.— Sound-mak- 
ing, 235  —The  sense  of  sight,  237. 

XIV. — INSTINCT  AND  REASON  .        .        .        .  •     .       .        .        .    240 

Irritability,  240.— Nerve  cells  and  fibers,  240.— The  brain  or 
sensorium,  241.— Reflex  action,  241.— Instinct,  242.— Classifica- 
tion of  instincts,  243.— Feeding,  244.— Self-defense,  245.— Play, 
247.— Climate,  248.— Environment,  248.— Courtship,  248.— Repro- 
duction, 249.— Care  of  the  young,  250.— Variability  of  instincts, 
251.— Reason,  251.— Mind,  255. 

XV. — HOMES  AND  DOMESTIC  HABITS 257 

Importance  of  care  of  the  young,  257. — Care  of  the  young  and 
communal  life,  257. — The  invertebrates  (except  spiders  and  in- 
sects), 258.— Spiders,  259.— Insects,  262.— The  vertebrates,  264. 

XVI. — GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS   ....    272 

Geographical  distribution,  272. — Laws  of  distribution,  274. — 
Species  debarred  by  barriers,  274.— Species  debarred  by  inability 
to  maintain  their  ground,  275.— Species  altered  by  adaptation  to 
new  conditions,  276.— Effect  of  barriers,  283.— Relation  of  species 
to  habitat,  283.— Character  of  barriers  to  distribution,  288.— Bar- 
riers affecting  fresh-water  animals,  294.— Modes  of  distribution, 
296.— Fauna  and  faunal  areas,  296.— Realms  of  animal  life,  297.— 
Subordinate  realms  or  provinces,  303.— Faunal  areas  of  the  sea, 
304. 

CLASSIFICATION  OF  ANIMALS       .       .       .       .       *       .    307 

GLOSSARY 313 

INDEX  .  .    319 


I 


ANIMAL  LIFE 


CHAPTEK  I 

THE   LIFE   OP   THE   SIMPLEST  ANIMALS 

1.  The  simplest  animals,  or  Protozoa. — The  simplest  ani- 
mals are  those  whose  bodies  are  simplest  in  structure  and 
which  do  the  things  done  by  all  living  animals,  such  as 
eating,  breathing,  moving,  feeling,  and  reproducing  in  the 
most  primitive  way.  The  body  of  a  horse,  made  up  of 
various  organs  and  tissues,  is  complexly  formed,  and  the 
various  organs  of  the  body  perform  the  various  kinds  of 
work  for  which  they  are  fitted  in  a  complex  way.  The 
simplest  animals  are  all  very  small,  and  almost  all  live  in 
the  water ;  some  kinds  in  fresh  water  and  many  kinds  in 
the  ocean.  Some  live  in  damp  sand  or  moss,  and  still  others 
are  parasites  in  the  bodies  of  other  animals.  They  are  not 
familiarly  known  to  us;  we  can  not  see  them  with  the 
unaided  eye,  and  yet  there  are  thousands  of  different  kinds 
of  them,  and  they  may  be  found  wherever  there  is  water. 

In  a  glass  of  water  taken  from  a  stagnant  pool  there 
is  a  host  of  animals.  There  may  be  a  few  water  beetles 
or  water  bugs  swimming  violently  about,  animals  half  an 
inch  long,  with  head  and  eyes  and  oar-like  legs ;  or  there 
may  be  a  little  fish,  or  some  tadpoles  and  wrigglers.  These 
are  evidently  not  the  simplest  animals.  There  will  be 
many  very  small  active  animals  barely  visible  to  the  un- 
aided eyes.  These,  too,  are  animals  of  considerable  com- 
plexity. But  if  a  single  drop  of  the  water  be  placed 
2  1 


ANIMAL  LIFE 


on  a  glass  slip  or  in  a  watch  glass  and  examined  with  a 
compound  microscope,  there  will  be  seen  a  number  of  ex- 
tremely small  creatures  which  swim  about  in  the  water-drop 
by  means  of  fine  hairs,  or  crawl  slowly  on  the  surface  of  the 
glass.  These  are  among  our  simplest  animals.  There  are, 
as  already  said,  many  kinds  of  these  "  simplest  animals," 
although,  perhaps  strictly  speaking,  only  one  kind  can  be 
called  simplest.  Some  of  these  kinds  are  spherical  in 
shape,  some  elliptical  or  football-shaped,  some  conical,  some 
flattened.  Some  have  many  fine,  minute  hairs  projecting 
from  the  surface  ;  some  have  a  few  longer,  stronger  hairs 
that  lash  back  and  forth  in  the  water,  and  some  have  no 
hairs  at  all.  There  are  many  kinds  and  they  differ  in  size, 
shape,  body  covering,  manner  of  movement,  and  habic  of 
food-getting.  And  some  are  truly  simpler  than  others. 
But  all  agree  in  one  thing  —  which  is  a  very  important 
thing  —  and  that  is  in  being  composed  in  the  simplest  way 
possible  among  animals. 

2.  The  animal  cell  —  The  whole  body  of  any  one  of  the 
simplest  animals  or  Protozoa  is  composed  for  the  animal's 
whole  lifetime  of  but  a  single  cell.  The  bodies  of  all  other 
animals  are  composed  of  many  cells.  The  cell  may  be 
called  the  unit  of  animal  (or  plant)  structure.  The  body 
of  a  h&se  is  complexly  composed  of  organs  and  tissues. 
Each  of  these  organs  and  tissues  is  in  turn  composed  of  a 
large  number  of  these  structural  units  called  cells.  These 
cells  are  of  great  variety  in  shape  and  size  and  general 
character.  The  cells  which  compose  muscular  tissue  are 
very  different  from  the  cells  which  compose  the  brain. 
And  both  of  these  kinds  of  cells  are  very  different  from 
the  simple  primitive,  undifferentiated  kind  of  cell  seen  in 
the  body  of  a  protozoan,  or  in  the  earliest  embryonic 
stages  of  a  many-celled  animal. 

The  animal  cell  is  rarely  typically  cellular  in  character 
—  that  is,  it  is  rarely  in  the  condition  of  a  tiny  sac  or  box 
of  symmetrical  shape.  Plant  cells  are  often  of  this  char- 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS  3 

acter.  The  primitive  animal  cell  (Fig.  1)  consists  of  a 
small  mass  of  a  viscid,  nearly  colorless,  substance  called 
protoplasm.  This  protoplasm  is  differentiated  to  form  two 
parts  or  regions  of  the  cell,  an  inner  denser  mass  called  the 
nucleus,  and  an  outer,  clearer,  inclosing  mass  called  the 
cytoplasm.  There  may  be  more  than 
one  nucleus  in  a  cell.  Sometimes 
the  cell  is  inclosed  by  a  cell  wall 
which  may  be  simply  a  tougher  outer 
layer  of  the  cytoplasm,  or  may  be  a 
thin  membrane  secreted  by  the  pro- 
toplasm. In  addition  to  the  proto- 
plasm, which  is  the  fundamental  and 
essential  cell  substance,  the  cell  may  FIG.  i.-Biood  ceil  of  a  crab 
contain  certain  so-called  cell  prod-  (after  HAECKEL).  show- 

,     .  .,          ..   ,       . ,       ,.„  ing  cytoplasm  and  nucleus 

ucts,  substances  produced  by  the  life        (the  large>  inner,  neariy 

processes    Of    the    protoplasm.        The  circular  spot)  and   gran- 

,,  . ,  .    .  .,  ules  of  various  substances 

cell   may  thus    contain  water,   oils,        ]ying  in  the  cytoplagm. 
resin,   starch  grains,  pigment  gran- 
ules, or  other  substances.      These  substances  are  held  in 
the  protoplasm  as  liquid  drops  or  solid  particles. 

The  protoplasm  itself  of  the  cell  shows  an  obvious 
division  into  parts,  so  that  certain  parts  of  it,  especially 
parts  in  the  nucleus,  have  received  names.  The  nucleus 
usually  has  a  thin  protoplasmic  membrane  surrounding  it, 
which  is  called  the  nuclear  membrane.  There  appear  to  be 
fine  threads  or  rods  in  the  nucleus  which  are  evidently 
different  from  the  rest  of  the  nuclear  protoplasm.  These 
rods  are  called  chromosomes.  The  cell  is,  indeed,  not  so 
simple  as  the  words  "  structural  unit "  might  imply,  but 
science  has  not  yet  so  well  analyzed  its  parts  as  to  warrant 
the  transfer  of  the  name  structural  unit  to  any  single  part 
of  the  cell— that  is,  to  any  lesser  or  simpler  part  of  the 
animal  body  than  the  cell  as  a  whole. 

The  protoplasm,  which  is  the  essential  substance  of  the 
cell  and  hence  of  the  whole  animal  body,  is  a  substance 


4  ANIMAL  LIFE 

of  a  very  complex  chemical  and  physical  constitution.  Its 
chemical  structure  is  so  complex  that  no  chemist  has  yet 
been  able  to  analyze  it,  and  as  the  further  the  attempts  at 
analysis  reach  the  more  complex  and  baffling  the  substance 
is  found  to  be,  it  is  not  improbable  that  it  may  never  be 
analyzed.  It  is  a  compound  of  numerous  substances,  some 
of  these  composing  substances  being  themselves  extremely 
complex.  The  most  important  thing  we  know  about  the 
chemical  constitution  of  protoplasm  is  that  there  are  al- 
ways present  in  it  certain  complex  albuminous  substances 
which  are  never  found  in  inorganic  bodies.  It  is  on  the 
presence  of  these  albuminous  substances  that  the  power  of 
performing  the  processes  of  life  depends.  Protoplasm  is  the 
primitive  basic  life  substance,  but  it  is  the  presence  of  these 
complex  albuminous  compounds  that  makes  protoplasm  the 
life  substance.  A  student  of  protoplasm  and  the  funda- 
mental life  processes,  Dr.  Davenport,  has  said,  "Just  as 
the  geologist  is  forced  by  the  facts  to  assume  a  vast  but 
not  infinite  time  for  earth  building,  so  the  biologist  has  to 
recognize  an  almost  unlimited  complexity  in  the  constitu- 
tion of  the  protoplasm."  * 


*  The  physical  structure  of  protoplasm  has  been  much  studied, 
but  even  with  the  improved  microscopes  and  other  instruments  neces- 
sary for  the  study  of  minute  structure,  naturalists  are  still  very  fat 
from  understanding  the  physical  constitution  of  this  substance.  While 
the  appearance  of  protoplasm  under  the  microscope  is  pretty  generally 
agreed  on  among  naturalists,  the  interpretation  of  the  kind  of  structure 
which  is  indicated  by  this  appearance  is  not  at  all  well  agreed  on. 
Protoplasm  appears  as  «,  mesh  work  composed  of  fine  granules  sus- 
pended in  a  clearer  substance,  the  spaces  of  the  mesh  work  being  com- 
posed of  a  third  still  clearer  substance.  Some  naturalists  believe,  from 
this  appearance,  that  protoplasm  is  composed  of  a  clear  viscous  sub- 
tance,  in  which  are  imbedded  many  fine  granules  of  denser  substance, 
and  numerous  large  globules  of  a  clearer,  more  liquid  substance.  Other 
naturalists  believe  that  the  fine  spots  which  appear  to  be  granules  are 
simply  cross  sections  of  fine  threads  of  dense  protoplasm  which  lie 
coiled  and  tangled  in  the  thinner,  clearer  protoplasm.  And,  finally, 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS  5 

3.  What  the  primitive  cell  can  do. — The  body  of  one  of 
the  minute  animals  in  the  water-drop  is  a  single  cell.     The 
body  is  not  composed  of  organs  of  different  parts,  as  in  the 
body  of  the  horse.     There  is  no  heart,  no  stomach  ;  there 
are  no  muscles,  no  nerves.     And  yet  the  protozoan  is  a  liv- 
ing animal  as  truly  as  is  the  horse,  and  it  breathes  and  eats 
and  moves  and  feels  and  produces  young  as  truly  as  does 
the  horse.     It  performs  all  the  processes  necessary  for  the 
life  of  an  animal.     The  single  cell,  the  single  minute  speck 
of  protoplasm,  has  the  power  of  doing,  in  a  very  simple  and 
primitive  way,  all  those  things  which  are  necessary  for 
life,  and  which  are  done  in  the  case  of  other  animals  by 
the  various  organs  of  the  body. 

4.  Amoeba. — The    simple   and   primitive  life  of   these 
Protozoa  can  be  best  understood  by  the   observation   of 
living  individuals.      In  the  slime   and  sediment  at  the 
bottom  of  stagnant  pools  lives  a  certain  specially  interest- 
ing kind  of  protozoan,  the  Amoeba  (Fig.  2).     Of  all  the 
simplest  animals  this  is  as  simple  or  primitive  as  any.    The 
minute  viscous  particle   of  protoplasm  which   forms   its 
body  is  irregular  in  outline,  and  its  outline  or  shape  slowly 
but  constantly  changes.    It  may  contract  into  a  tiny  ball ; 
it  may  become  almost  star-shaped  ;  it  may  become  elongate 
or  flattened ;   short,  blunt,  finger-like  projections  called 
pseudppods  extend  from  the  central  body  mass,  and  these 
projections  are  constantly  changing,  slowly  pushing  out  or 

others  believe  that  protoplasm  exists  as  a  foam  work ;  that  it  is  a  vis- 
cous liquid  containing  many  fine  globules  (the  granule-appearing  spots) 
of  a  liquid  of  different  density  and  numerous  larger  globules  of  a  liquid 
of  still  other  density.  It  is  a  foam  in  which  the  bubbles  are  not  filled 
with  air,  but  with  liquids  of  different  density.  This  last  theory  of  the 
structure  of  protoplasm  is  the  one  accepted  by  a  majority  of  modern 
naturalists,  although  the  other  theories  have  numerous  believers.  But 
just  as  with  what  little  we  know  of  the  chemical  constitution  of  proto- 
plasm, the  little  we  know  of  its  physical  structure  throws  almost  no 
light  on  the  remarkable  properties  of  this  fundamental  life  substance. 
2 


6 


ANIMAL  LIFE 


drawing  in.  The  single  protoplasmic  cell  which  makes  up 
the  body  of  the  Amoeba  has  no  fixed  outline ;  it  is  a  cell 
without  a  wall.  The  substance  of  the  cell  or  body  is  proto- 
plasm, semiliquid  and  colorless.  The  changes  in  form  of 
the  body  are  the  moving  of  the  Amoeba.  By  close  watching 
it  may  be  seen  that  the  Amoeba  changes  its  position  on  the 
glass  slip.  Although  provided  with  no  legs  or  wings  or 


Fio.  2.— An  Amoeba,  showing  different  shapes  assumed  by  it  when  crawling. 
—After  VERWORN. 

scales  or  hooks — that  is,  with  no  special  organs  of  locomo- 
tion— the  Amoeba  moves.  There  are  no  muscles  in  this  tiny 
"body;  muscles  are  composed  of  many  contractile  cells 
massed  together,  and  the  Amoeba  is  but  one  cell.  But  it  is 
a  contractile  cell ;  it  can  do  what  the  muscles  of  the  com- 
plex animals  do. 

If  one  of  the  finger-like  projections  of  the  Amoeba^  or, 
indeed,  if  any  part  of  its  body  comes  in  contact  with  some 
other  microscopic  animal  or  plant  or  some  small  fragment 
of  a  larger  form,  the  soft  body  of  the  Amoeba  will  be  seen 


THE  LIFE  OF   THE  SIMPLEST  ANIMALS 


to  press  against  it,  and  soon  the  plant  or  animal  or  organic 
particle  becomes  sunken  in  the  protoplasm  of  the  formless 
body  and  entirely  inclosed  in  it  (Fig.  3).  The  absorbed 
particle  soon  wholly  or  partly  disappears.  This  is  the 
manner  in  which  the  Amoeba  eats.  It  has  no  mouth  or 


e  u. 


FIG.  3.— Amoeba  eating  a  microscopic  one-celled  plant.— After  VERWOKN. 

stomach.  Any  part  of  its  body  mass  can  take  in  and  digest 
food.  The  viscous,  membraneless  body  simply  flows  about 
the  food  and  absorbs  it.  Such  of  the  food  particles  as  can 
not  be  digested  are  thrust  out  of  the  body. 

The  Amoeba  breathes.  Though  we  can  not  readily  ob- 
serve this  act  of  respiration,  it  is  true  that  the  Amoeba  takes 
into  its  body  through  any  part  of  its  surface  oxygen  from 
the  air  which  is  mixed  with  water,  and  it  gives  off  from  any 
part  of  its  body  carbonic-acid  gas.  Although  the  Amoeba 
has  no  lungs  or  gills  or  other  special  organs  of  respiration, 
it  breathes  in  oxygen  and  gives  out  carbonic-acid_gas,  which 
is  just  what  the  horse  does  with  its  elaborately  developed 
organs  of  respiration. 

If  the  Amoeba,  in  moving  slowly  about,  comes  into  con- 
tact with  a  sand  grain  or  other  foreign  particle  not  suitable 
for  food,  the  soft  body  slowly  recoils  and  flows — for  the 
movement  is  really  a  flowing  of  the  thickly  fluid  protoplasm 
— so  as  to  leave  the  sand  grain  at  one  side.  The  Amoeba 
feels.  It  shows  the  effects  of  stimulation.  Its  movements 
can  be  changed,  stopped,  or  induced  by  mechanical  or 
chemical  stimuli  or  by  changes  in  temperature.  The 


8 


ANIMAL  LIFE 


Amoeba  is  irritable ;  it  possesses  irritability,  which  is  sensa- 
tion in  its  simplest  degree. 

If  food  is  abundant  the  Amoeba  soon  increases  in  size. 
The  bulk  of  its  body  is  bound  to  increase  if  new  substance 


FIG.  4.—Amceba  polypodia  in  six  successive  stages  of  division.     The  dark,  white- 
margined  spot  in  the  interior  is  the  nucleus.— After  F.  E.  SCHULZE. 

is  constantly  assimilated  and  added  to  it.  The  Amoeba 
grows.  But  there  seem  to  be  some  fixed  limits  to  the 
extent  of  this  increase  in  size.  No  Amceba  becomes  large. 
A  remarkable  phenomenon  always  occurs  to  prevent  this. 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS  9 

An  Amoeba  which  has  grown  for  some  time  contracts  all 
its  finger-like  processes,  and  its  body  becomes  constricted. 
This  constriction  or  fissure  increases  inward,  so  that  the 
body  is  soon  divided  fairly  in  two  (Fig.  4).  The  body, 
being  an  animal  cell,  possesses  a  nucleus  imbedded  in  the 
body  protoplasm  or  cytoplasm.  When  the  body  begins  to 
divide,  the  nucleus  begins  to  divide  also,  and  becomes  en- 
tirely divided  before  the  fission  of  the  cytoplasm  is  com- 
plete. There  are  now  two  Amoeba,  each  half  the  size  of 
the  original  one ;  each,  indeed,  being  actually  one  half  of 
the  original  one.  This  splitting  of  the  body  of  the  Amoeba, 
which  is  called  fission,  is  the  process  of  reproduction.  The 
original  Amoeba  is  the  parent ;  the  two  halves  of  the  parent 
are  the  young.  Each  of  the  young  possesses  all  of  the 
characteristics  and  powers  of  the  parent ;  each  can  move, 
eat,  feel,  grow,  and  reproduce  by  fission.  It  is  very  evident 
that  this  is  so,  for  any  part  of  the  body  or  the  whole  body 
was  used  in  performing  these  functions,  and  the  young  are 
simply  two  parts  of  the  parent's  body.  But  if  there  be  any 
doubt  about  the  matter,  observation  of  the  behavior  of  the 
young  or  new  Amcebce,  will  soon  remove  it.  Each  puts  out 
pseudopods,  moves,  ingests  food  particles,  avoids  sand 
grains,  contracts  if  the  water  is  heated,  grows,  and  finally 
divides  in  two. 

5.  Paramoecium. — Another  protozoan  which  is  common 
in  stagnant  pools  and  can  be  readily  obtained  and  observed 
is  Paramoscium  (Fig.  5).  The  body  of  the  Paramoecium  is 
much  larger  than  that  of  the  Amoeba,  being  nearly  one  fourth 
of  a  millimeter  in  length,  and  is  of  fixed  shape.  It  is  elon- 
gate, elliptical,  and  flattened,  and  when  examined  under  the 
microscope  seems  to  be  a  very  complexly  formed  little  mass. 
The  body  of  the  Paramoecium  is  indeed  less  primitive  than 
that  of  the  Amoeba,  and  yet  it  is  still  but  a  single  cell. 
The  protoplasm  of  the  body  is  very  soft  within  and  dense 
on  the  outside,  and  it  is  covered  externally  by  a  thin  mem- 
brane. The  body  is  covered  with  short  fine  hairs  or  cilia, 


10 


ANIMAL  LIFE 


which  are  fine  processes  of  the  dense  protoplasm  of  the 
surface.  There  is  on  one  side  an  oblique  shallow  groove 
that  leads  to  a  small,  funnel-shaped  depression  in  the  body 
which  serves  as  a  primitive  sort  of  mouth 
or  opening  for  the  ingress  of  food. 
The  Paramcecium  swims  about  in  the 
water  by  vibrating  the  cilia  which  cov- 
er the  body,  and  brings  food  to  the 
mouth  opening  by  producing  tiny  cur- 
rents in  the  water  by  means  of  the 
cilia  in  the  oblique  groove.  The  food, 
which  consists  of  other  living  Proto- 
zoa, is  taken  into  the  body  mass  only 
through  the  funnel-shaped  opening,  and 
that  part  of  it  which  is  undigested  is 
thrust  out  always  through  a  particular 
part  of  the  body  surface.  (The  taking 
in  and  ejecting  of  foreign  particles  can 
be  seen  by  putting  a  little  powdered 
carmine  in  the  water.)  Within  the 
body  there  are  two  nuclei  and  two  so- 
called  pulsating  vacuoles.  These  pul- 
FiQ.5.—paramaciumau-  sating  vacuoles  (Amoeba  has  one)  seem 

relia  (after  VKKWORN).  .  ,    .        , .      ,          .  , 

At  each  end  there  is  a      to    aid  m   discharging    Waste    products 

contractile  vacuoie,  and    from  the  body.      When   the  Paramoe- 

in  the  center  is  one  of         •  •>  •        •  i     , 

the  nuclei  cium  touches  some  foreign  substance  or 

is  otherwise  irritated  it  swims  away, 
and  it  shoots  out  from  the  surface  of  its  body  some  fine 
long  threads  which  when  at  rest  are  probably  coiled  up  in 
little  sacs  on  the  surface  of  the  body.  When  the  Para- 
mcecium has  taken  in  enough  food  and  grown  so  that  it 
has  reached  the  limit  of  its  size,  it  divides  transversely  into 
halves  as  the  Amoeba  does.  Both  nuclei  divide  first,  and 
then  the  cytoplasm  constricts  and  divides  (Fig.  6).  Thus 
two  new  Paramoecia  are  formed.  One  of  them  has  to  de- 
velop a  new  mouth  opening  and  groove,  so  that  there  is  in 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS 


11 


the  case  of  the  reproduction  of  Paramoscium  the  beginnings 
of  developmental  changes  during  the  course  of  the  growth 
of  the  young.  The  young  Amcebcs  have  only  to  add  sub- 
stance to  their  bodies,  to  grow  larger,  in  order  to  be  exactly 
like  their  parent. 

The  new  Paramcecia  attain  full  size  and  then  divide, 
each  into  two.  And  so  on  for  many  generations.  But  it 
has  been  discovered  that  this  simplest  kind  of  reproduction 
can  not  go  on  indefinitely.  After  a  number  of  generations 
the  Paramcecia,  instead  of  simply  dividing  in  two,  come 

together  in  pairs,  and  a  part  of 
one  of  the  nuclei  of  each  mem- 
ber of  a  pair  passes  into  the 
body  of  and  fuses  with  a  part 


FIG.  6.— Paramcecium  putorinum 
dividing.  The  two  nuclei  be- 
come very  elongate  before  di- 
viding.—After  BUTSCHLI. 


PIG.  7. — Paramcecium  caudatum ;  two  indi- 
viduals separating  after  conjugation. 


of  one  of  the  nuclei  of  the  other  member  of  the  pair.  In 
the  meantime  the  second  nucleus  in  each  Paramoscium  has 
broken  up  into  small  pieces  and  disappeared.  The  new 
nucleus  composed  of  parts  of  the  nuclei  from  two  animals 
divides,  giving  each  animal  two  nuclei  just  as  it  had  before 
this  extraordinary  process,  which  is  called  conjugation, 
began  (Fig.  7).  Each  Paramcecium,  with  its  nuclei  com- 
posed of  parts  of  the  nuclei  from  two  distinct  individuals. 


12 


ANIMAL  LIFE 


now  simply  divides  in  two,  and  a  large  number  of  genera- 
tions by  simple  fission  follow. 

Paramwcium  in  the  character  of  its  body  and  in  the 
manner  of  the  performance  of  its  life  processes  is  distinctly 
less  simple  than  the  Amoeba,  but  its  body  is  composed  of  a 
single  structural  unit,  a  single  cell,  and  it  is  truly  one  of 
the  "  simplest  animals." 

6.  Vorticella. — Another  interesting  and  readily  found 
protozoan  is  Vorticella  (Fig.  8).  While  the  Amoeba  can  crawl 
and  ParamoBcium  swim,  Vorticella,  except  when  very  young, 


FIG.  8. —  VorticeUa  microstoma  (after  STEIN).  A,  small,  free-swimming  individuals 
conjugating  with  a  large,  stalked  individual ;  B,  a  stalked  individual  dividing 
longitudinally ;  C,  after  division  is  completed  one  part  severs  itself  from  the 
other,  forms  a  ring  of  cilia,  and  swims  away. 

is  attached  by  tiny  stems  to  dead  leaves  or  sticks  in  the 
water,  and  can  change  its  position  only  to  a  limited  extent. 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS  13 

The  body  is  pear-shaped  or  bell-shaped,  with  a  mouth 
opening  at  the  broad  end,  and  a  delicate  stem  at  the 
narrow  end.  This  stem  is  either  hard  and  stiff,  or  is 
flexible  and  capable  of  being  suddenly  contracted  in  a 
close  spiral.  In  the  body  mass  there  is  one  pulsating 
vacuole  and  one  nucleus.  Usually  many  Vorticellce  are 
found  together  on  a  common  stalk,  thus  forming  a  proto- 
zoan colony. 

The  life  processes  of  Vorticella  are  of  the  simple  kind 
already  observed  in  Amwba  and  Paramwcium.  Vorticella 
shows,  however,  some  modifications  of  the  process  of  repro- 
duction which  are  interesting.  The  plane  of  division  of 
Vorticella  is  parallel  to  the  long  axis  of  the  pear-shaped 
body,  so  that  when  fission  is  complete  there  are  two  Vorti- 
cellce  on  a  single  stalk.  One  of  the  two  becomes  detached, 
and  by  means  of  a  circle  of  fine  hairs  or  cilia  which  appear 
around  its  basal  end  leads  a  free  swimming  life  for  a  short 
time.  Finally  it  settles  down  and  develops  a  stalk.  Vorti- 
cella shows  two  kinds  of  fission — one  the  usual  division 
into  equal  parts,  and  another  division  into  unequal  parts. 
In  this  latter  kind,  called  reproduction  or  multiplication 
by  budding,  a  small  part  of  the  parent  body  separates, 
develops  a  basal  circle  of  cilia,  and  swims  away.  The  pro- 
cess of  conjugation  also  takes  place  among  the  Vorti- 
cella, but  they  are  never  two  equal  forms  which  conju- 
gate, but  always  one  of  the  ordinary  stalked  forms  and 
one  of  the  small  free  -  swimming  forms  produced  by 
budding. 

Here,  then,  in  the  life  of  Vorticella,  are  new  modifica- 
tions of  the  life  processes  ;  but,  after  all,  these  life  processes 
are  very  simply  performed,  and  the  body  is  like  the  body  of 
the  Amoeba,  a  single  cell.  Vorticella  is  plainly  one  of  "  the 
simplest  animals." 

7.  Gregarina. — A  fourth  kind  of  protozoan  to  which  we 
can  profitably  give  some  special  attention  is  Gregarina 
(Fig.  9),  the  various  species  of  which  live  in  the  alimentary 


ANIMAL  LIFE 


canal*  of  crayfishes  and  centipeds  and  certain  insects. 
Gregarina  is  a  parasite,  living  at  the  expense  of  the  host 
in  whose  body  it  lies.  It  has  no  need  to  swim  about  quickly, 


B 


FIG.  9.— Gregarinidse.  A,  a  Gregarinid  (Actinocephalus  oligacanthus)  from  the  intes- 
tine of  an  insect  (after  STEIN)  ;  B  and  C,  spore  forming  by  a  Gregarinid  (Coc- 
cidium  oviforme)  from  the  liver  of  a  guinea-pig  (after  LEUCKART)  ;  D,  E,  and 
F,  successive  stages  in  the  conjugation  and  spore  forming  of  Gregarina  poly- 
morpha  (after  KOBLLIKER). 

and  hence  has  no  swimming  cilia  like  Paramcecium  and 
the  young  Vorticella.  It  does  need  to  cling  to  the  inner 
wall  of  the  alimentary  canal  of  its  host,  and  the  body  of 
some  species  is  provided  with  hooks  for  that  purpose.  The 

*  Specimens  of  Gregarina  can  be  abundantly  found  in  the  alimen- 
tary canal  of  meal  worms,  the  larvae  of  the  black  beetle  (Tenebrio  moli- 
tor),  common  in  granaries,  mills,  and  brans.  "Snip  off  with  small 
scissors  both  ends  of  a  larva,  seize  the  protruding  (white)  intestine  with 
forceps,  draw  it  out,  and  tease  a  portion  in  normal  salt  solution  (water 
will  do)  on  a  slide.  Cover,  find  with  the  low  power  (minute,  oblong, 
transparent  bodies),  and  study  with  any  higher  objective  to  suit." — 
MURBACH. 


THE  LIFE  OF   THE  SIMPLEST  ANIMALS  15 

food  of  Gregarina  is  the  liquid  food  of  the  host  as  it  exists 
in  the  intestine,  and  which  is  simply  absorbed  anywhere 
through  the  surface  of  the  body  of  the  parasite.  There  is 
no  mouth  opening  nor  fixed  point  of  ejection  of  waste 
material,  nor  is  there  any  contractile  vacuole  in  the  body. 

In  the  method  of  multiplication  or  reproduction  Gre- 
garina shows  an  interesting  difference  from  Amoeba  and 
Paramwcium  and  Vorticella.  When  the  Gregarina  is 
ready  to  multiply,  its  body,  which  in  most  species  is  rather 
elongate  and  flattened,  contracts  into  a  ball-shaped  mass 
and  becomes  encysted — that  is,  becomes  inclosed  in  a  tough, 
membranous  coat.  This  may  in  turn  be  covered  externally 
by  a  jelly-like  substance.  The  nucleus  and  the  protoplasm 
of  the  body  inside  of  the  coat  now  divide  into  many  small 
parts  called  spores,  each  spore  consisting  of  a  bit  of  the 
cytoplasm  inclosing  a  small  part  of  the  original  nucleus. 
Later  the  tough  outer  wall  of  the  cyst  breaks  and  the 
spores  fall  out,  each  to  grow  and  develop  into  a  new  Gre- 
garina. In  some  species  there  are  fine  ducts  or  canals 
leading  from  the  center  of  the  cyst  through  the  wall  to  the 
outside,  and  through  these  canals  the  spores  issue.  Some- 
times two  GregarincB  come  together  before  encystation  and 
become  inclosed  in  a  common  wall,  the  two  thus  forming  a 
single  cyst.  This  is  a  kind  of  conjugation.  In  some  spe- 
cies each  of  the  young  or  new  GregarincB  coming  from  the 
spores  immediately  divides  by  fission  to  form  two  indi- 
viduals. 

8.  Marine  Protozoa. — If  called  upon  to  name  the  char- 
acteristic animals  of  the  ocean,  we  answer  readily  with  the 
names  of  the  better-known  ocean  fishes,  like  the  herring  and 
cod,  which  we  know  to  live  there  in  enormous  numbers ;  the 
seals  and  sea  lions,  the  whales  and  porpoises,  those  fish-like 
animals  which  are  really  more  like  land  animals  than  like 
the  true  fishes ;  and  the  jelly-fishes  and  corals  and  star-fishes 
which  abound  along  the  ocean's  edge.  But  in  naming  only 
these  we  should  be  omitting  certain  animals  which  in  point 


16  ANIMAL  LIFE 

of  abundance  of  individuals  vastly  outnumber  all  other 
animals,  and  which  in  point  of  importance  in  helping  main- 
tain the  complex  and  varied  life  of  the  ocean  distinctly  out- 
class all  other  marine  forms.  These  animals  are  the  marine 
Protozoa,  those  of  the  "  simplest  animals  "  which  live  in  the 
ocean. 

Although  the  water  at  the  surface  of  the  ocean  appears 
clear,  and  on  superficial  examination  devoid  of  life,  yet  a 
drop  of  this  water  taken  from  certain  ocean  regions  exam- 
ined under  the  microscope  reveals  the  fact  that  this  water 
is  inhabited  by  Protozoa.  Not  only  is  the  water  at  the 
very  surface  of  the  ocean  the  home  of  the  simplest  animals, 
but  they  can  be  found  in  all  the  water  from  the  surface  to 
a  great  depth  beneath  it.  In  a  pint  of  this  ocean  water 
from  the  surface  or  near  it  there  may  be  millions  of  these 
animals.  In  the  oceans  of  the  world  the  number  of  them 
is  inconceivable.  Dr.  W.  K.  Brooks  says  that  the  "  basis 
of  all  the  life  in  the  modern  ocean  is  found  in  the  micro- 
organisms of  the  surface."  By  micro-organisms  he  means 
the  one-celled  animals  and  the  one-celled  plants.  For 
the  simplest  plants  are,  like  the  simplest  animals,  one- 
celled.  "  Modern  microscopical  research,"  he  says,  "  has 
shown  that  these  simple  plants,  and  the  Globigerinse  and 
Kadiolaria  [kinds  of  Protozoa]  which  feed  upon  them,  are 
so  abundant  and  prolific  that  they  meet  all  demands  and 
supply  the  food  for  all  the  animals  of  the  ocean." 

9.  The  Globigerinse  and  Radiolaria.— The  Globigerinas 
(Fig.  10)  and  Radiolaria  (Fig.  11)  are  among  the  most  in- 
teresting of  all  the  simplest  animals.  Their  simple  one- 
celled  body  is  surrounded  by  a  microscopic  shell,  which 
among  the  Globigerinae  is  usually  made  of  lime  (calcium 
carbonate),  in  the  case  of  Radiolaria  of  silica.  These  minute 
shells  present  a  great  variety  of  shape  and  pattern,  many 
being  of  the  most  exquisite  symmetry  and  beauty.  The 
shells  are  usually  perforated  by  many  small  holes,  through 
which  project  long,  delicate,  protoplasmic  threads.  These 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS 


17 


fine  threads  interlace  when  they  touch  each  other,  thus 
forming  a  sort  of  protoplasmic  network  outside  of  the  shell. 
In  some  cases  there  is  a  complete  layer  of  protoplasm — 
part  of  the  body  protoplasm  of  the  protozoan  — surround- 


PIG.  W.—PolystomeUa  strigittata,  one  of  the  Globigerinte.—  After  MAX  SCHULTZB. 

ing  the  cell  externally.  The  Radiolaria,  whose  shells  are 
made  of  silica,  possess  also  a  perforated  membranous  sac 
called  the  central  capsule,  which  lies  imbedded  in  the 
protoplasm,  dividing  it  into  two  portions,  one  within  and 
3 


18  ANIMAL  LIFE 

one  outside  of  the  capsule.  In  the  protoplasm  inside  of 
the  capsule  lies  the  nucleus  or  nuclei ;  and  from  the  proto- 
plasm outside  of  the  capsule  rise  the  numerous  fine,  thread- 
like pseudopods  which  project  through  the  apertures  in  the 
shell,  and  enable  the  animal  to  swim  and  to  get  food. 

Most  of  the  myriads  of  the  simplest  animals  which 
swarm  in  the  surface  waters  of  the  ocean  belong  to  a  few 
kinds  of  these  shell-bearing  Globigerinae  and  Radiolaria. 
Large  areas  of  the  bottom  of  the  Atlantic  Ocean  are  cov- 
ered with  a  slimy  gray  mud,  often  of  great  thickness,  which 
is  called  globigerina-ooze,  because  it  is  made  up  chiefly  of 
the  microscopic  shells  of  Globigerinae.  As  death  comes  to 
the  minute  protoplasmic  animals  their  hard  shells  sink 
slowly  to  the  bottom,  and  accumulate  in  such  vast  quanti- 
ties as  to  form  a  thick  layer  on  the  ocean  floor.  Nor  is  it 
only  in  present  times  and  in  the  oceans  we  know  that  the 
Globigerinae  have  flourished.  All  over  the  world  there  are 
thick  rock  strata  which  are  composed  chiefly  of  the  fos- 
silized shells  of  these  simplest  animals.  Where  the  strata 
are  made  up  exclusively  of  these  shells  the  rock  is  chalk. 
Thus  are  composed  the  great  chalk  cliffs  of  Kent,  which 
gave  to  England  the  early  name  of  Albion,  and  the  chalk 
beds  of  France  and  Spain  and  Greece.  The  existence  of 
these  chalk  strata  means  thai'Vher^  now  is  land,  in  earlier 
geologic  times  were  oceans,  and  that  in  the  oceans  Globi- 
gerinae lived  in  countless  numbers.  Dying,  their  shells 
accumulated  to  form  thick  layers  on  the  sea  bottom.  In 
later  geologic  ages  this  sea  bottom  has  been  uplifted  and 
is  now  land,  far  perhaps  from  any  ocean.  The  chalk  strata 
of  the  plains  of  the  United  States,  like  those  in  Kansas,  are 
more  than  a  thousand  miles  from  the  sea,  and  yet  they  are 
mainly  composed  of  the  fossilized  shells  of  marine  Pro- 
tozoa. Indeed,  we  are  acquainted  with  more  than  twice  as 
many  fossil  species  of  Globigerinae  as  species  living  at  the 
present  time.  The  ancestors  of  these  Globigerinae,  from 
which  the  present  Globigerinae  differ. -but  little,  can  be* 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS 


19 
It  is 


traced  far  back  in  the  geologic  history  of  the  world, 
an  ancient  type  of  animal  structure. 

The  Kadiolaria,  too,  which  live  abundantly  in  the  pres- 
ent oceans,  especially  in  the  marine  waters  of  the  tropical 
and  temperate  zones,  are  found  as  fossils  in  the  rocks  from 
the  time  of  the  coal  age  on.  The  siliceous  shells  of  the 


PIG.  ll.—Heliosphcera  actinota  (after  HAECKEL)  ;    a  radiolarian  with  symmetrical 

shell. 

Radiolaria  sinking  to  the  sea  bottom  and  accumulating 
there  in  great  masses  form  a  radiolaria-ooze  similar  to  the 
globigerinae-ooze ;  and  just  as  with  the  Globigerinae,  the 
remains  of  the  ancient  Radiolaria  formed  thick  layers  on 
the  floor  of  the  ancient  oceans,  which  have  since  been  up- 
lifted and  now  form  certain  rock  strata.  That  kind  of 
bock  called  Tripoli,  found  in  Sicily,  and  the  Barbados 
earth  from  the  island  of  Barbados,  both  of  which  are  used 


20  ANIMAL  LIFE 

as  polishing  powder,  are  composed  almost  exclusively  of 
the  siliceous  shells  of  ancient  and  long-extinct  Radiolaria. 

10.  Antiquity  of  the  Protozoa. — All  the  animals  of  the 
ocean  depend  upon  the  marine  Protozoa  (and  the  marine 
Protophyta,  or  one-celled  plants)  for  food.     Either  they 
prey  upon  these  one-celled  organisms  directly,  or  they  prey 
upon  animals  which  do  prey  on  these  simplest  animals. 
The  great  zoologist  already  quoted  says :  "  The  food  sup- 
ply of  marine  animals  consists  of  a  few  species  of  micro- 
scopic   organisms  which  are  inexhaustible   and  the   only 
source  of  food  for  all  the  inhabitants  of  the  ocean.     The 
supply  is  primeval  as  well  as  inexhaustible,  and  all  the  life 
of  the  ocean  has  gradually  taken  shape  in  direct  depend- 
ence upon  it."     That  is,  the  marine  simplest  animals  are 
the  only  marine  animals  which  live  independently;  they 

/^lone  can  live  or  could  have  lived  in  earlier  ages  without 
jp      depending  on  other  animals.     They  must  therefore  be  the 
'oldest  of  marine  animals.     By  oldest  we  mean  that  their 
kind  appeared  earliest  in  the  history  of  the  world.     As  it 
is  certain  that  marine  life  is  older  than  terrestrial  life — that 
is,  that  the  first  animals  lived  in  the  ocean — it  is  obvious 
that  the  marine  Protozoa  are  the  most  ancient  of  animals. 
This  is  an  important  and  interesting  fact.     Zoologists  try 
to  find  out  the  relationships  and  the  degrees  of  antiquity 
or  modernness  of  the  various  kinds  of  animals.     We  have 
seen  that  the  Protozoa,  those  animals  which  have  the  sim- 
\    pleat  body  structure  and  perform  the  necessary  life  pro- 
cesses in  the  simplest  way,  are  the  oldest,  the  first  animals. 
L  Tfrs  is  just  what  we  would  expect. 

11.  The  primitive  form. — We  find  among  the  simplest 
animals  a  considerable  variety  of  shape  and  some  manifest 
variation  in  habit.     But  the  points  of  resemblance  are  far 
more  pronounced  than  the  points  of  difference,  and  are  of 
fundamental  importance.     The  composition  of  the  body  of 
one  cell,  as  opposed  to  the  many-celled  structure  of  the 
bodies  of  all  other  animals,  is  the  fact  to  be  most  distinctly 


THE  LIFE  OF  THE  SIMPLEST  ANIMALS  21 

emphasized.  The  shape  of  this  one-celled  body  varies. 
With  the  most  primitive  or  simplest  of  the  "  simplest  ani- 
mals," like  Amoeba,  -for  example,  there  is  no  "  distinction 
of  ends,  sides,  or  surfaces,  such  as  we  are  familiar  with  in 
in  the  higher  animals.  Anterior  and  posterior  ends,  right 
and  left  sides,  dorsal  and  ventral  surfaces  are  terms  which 
have  no  meaning  in  reference  to  an  Amoeba,  for  any  part 
of  the  animal  may  go  first  in  locomotion,  and  when  crawl- 
ing the  animal  moves  along  on  whatever  part  of  its 
surface  happens  to  be  in  contact  with  foreign  bodies." 
The  one  shape  most  often  seen  among  the  Protozoa,  or 
most  nearly  fairly  to  be  called  the  typical  shape,  is  the 
spherical  or  subspherical  shape.  Why  this  is  so  is  readily 
seen.  Most  of  the  Protozoa  are  aquatic  and  free  swim- 
ming. They  live  in  a  medium,  the  water,  which  supports 
or  presses  on  the  body  equally  on  all  sides,  and  the  body  is 
not  forced  to  assume  any  particular  form  by  the  environ- 
ment. The  body  rests  suspended  in  the  water  with  any 
part  of  its  surface  uppermost  or  any  part  undermost.  As 
any  part  of  the  surface  serves  equally  well  in  many  of  the 
Protozoa  for  breathing  or  eating  or  excreting,  it  is  obvious 
that  the  spherical  form  is  the  simplest  and  most  conven- 
ient shape  for  such  a  body.  It  is  interesting  to  note  that 
the  spherical  form  is  the  common  shape  of  the  egg  cell  of 
the  higher  animals.  Each  one  of  the  higher,  multicellular 
animals  begins  life  (as  we  shall  find  it  explained  in  another 
chapter  of  this  book)  as  a  single  cell,  the  egg  cell,  and 
these  egg  cells  are  usually  spherical  in  shape.  The  full 
significance  of  this  we  need  not  now  attempt  to  under- 
stand, but  it  is  interesting  to  note  that  normally  the  whole 
body  of  the  simplest  animals  is  a  single  spherical  cell,  and 
that  every  one  of  the  higher  animals,  however  complex 
it  may  become  by  growth  and  development,  begins  life  as  a 
single  spherical  cell. 

12.  The  primitive  but  successful  life. — Living  consists  of 
the  performing  of  certain  so-called  life  processes,  such  as 
3 


22  ANIMAL  LIFE 

eating,  breathing,  feeling,  and  multiplying.  These  pro- 
cesses are  performed  among  the  higher  animals  by  various 
organs,  special  parts  of  the  body,  each  of  which  is  fitted  to 
do  some  one  kind  of  work,  to  perform  some  one  of  these 
processes.  There  is  a  division  or  assignment  of  labor  here 
among  different  parts  of  the  body.  Such  a  division  of 
labor,  and  special  fitting  of  different  parts  of  the  body  for 
special  kinds  of  work  does  not  exist,  or  exists  only  in 
slightest  degree  among  the  simplest  animals.  The  Amo&ba 
eats  or  feels  or  moves  with  any  part  of  its  body ;  all  of  the 
body  exposed  to  the  air  (air  held  in  the  water)  breathes ; 
the  whole  body  mass  takes  part  in  the  process  of  repro- 
duction. 

Only  very  small  organisms  can  live  in  this  simplest  way. 
So  all  of  the  Protozoa  are  minute.  When  the  only  part  of 
the  body  which  can  absorb  oxygen  is  the  simple  external 
surface  of  a  spherical  body,  the  mass  of  that  body  must  be 
very  small.  With  any  increase  in  size  of  the  animal  the 
mass  of  the  body  increases  as  the  cube  of  the  diameter, 
while  the  surface  increases  only  as  the  square  of  the  diam- 
eter. Therefore  the  part  of  the  body  (inside)  which  re- 
quires to  be  provided  with  oxygen  increases  more  rapidly 
than  the  part  (the  outside)  which  absorbs  oxygen.  Thus 
this  need  of  oxygen  alone  is  sufficient  to  determine  the 
limit  of  size  which  can  be  attained  by  the  spherical  or  sub- 
spherical  Protozoa. 

That  the  simplest  animals,  despite  the  lack  of  organs 
and  the  primitive  way  of  performing  the  life  processes,  live 
successfully  is  evident  from  their  existence  in  such  ex- 
traordinary numbers.  They  outnumber  all  other  animals. 
Although  serving  as  food  for  hosts  of  ocean  animals,  the 
marine  Protozoa  are  the  most  abundant  in  individuals  of 
all  living  animals.  The  conditions  of  life  in  the  surface 
waters  of  the  ocean  are  easy,  and  a  simple  structure  and 
simple  method  of  performance  of  the  life  processes  are 
wholly  adequate  for  successful  life  under  these  conditions, 


THE  LIFE   OF   THE  SIMPLEST  ANIMALS  23 

That  the  character  of  the  body  structure  of  the  Protozoa 
has  changed  but  little  since  early  geologic  times  is  ex- 
plained by  the  even,  unchanging  character  of  their  sur-  ,/ 
roundings.  The  oceans  of  former  ages  have  undoubtedly 
been  essentially  like  the  oceans  of  to-day — not  in  extent 
and  position,  but  in  their  character  of  place  of  habitation 
for  animals.  The  environment  is  so  simple  and  uniform 
that  there  is  little  demand  for  diversity  of  habits  and  conse- 
quent diversity  of  body  structure.  Where  life  is  easy  there 
is  no  necessity  for  complex  structure  or  complicated  habits 
of  living.  So  the  simplest  animals,  unseen  by  us,  and  so 
inferior  to  us  in  elaborateness  of  body  structure  and  habit, 
swarm  in  countless  hordes  in  all  the  oceans  and  rivers  and 
*akes,  and  live  successfully  their  simple  lives. 


CHAPTEE  II 

THE  LIFE  OP  THE  SLIGHTLY  COMPLEX  ANIMALS 

13.  Colonial  Protozoa. — When  one  of  the  simplest  animals 
multiplies  by  fission,  the  halves  of  the  one-celled  body  sepa- 
rate wholly  from  each  other,  move  apart,  and  pursue  their 
lives  independently.  The  original  parent  cell  divides  to 
form  two  cells,  which  exist  thereafter  wholly  apart  from 
each  other.  There  are,  however,  certain  simple  animals 
which  are  classed  with  the  Protozoa,  which  show  an  inter- 
esting and  important  difference  from  the  great  majority  of 
the  simplest  animals.  These  are  the  so-called  colony-form- 
ing or  colonial  Protozoa. 

These  colonial  Protozoa  belong  to  a  group  of  organisms 
called  the  *  Volvocinae.  The  simplest  of  the  Volvocinae  are 
single  cells,  which  live  wholly  independently  and  are  in 
structure  and  habit  essentially  like  the  other  Protozoa  we 
have  studied.  They  have,  however,  imbedded  in  the  one- 
celled  body  a  bit  of  chlorophyll,  the  green  substance  which 
gives  the  color  to  green  plants  and  is  so  important  in  their 
physiology.  In  this  respect  they  differ  from  the  other 
Protozoa.  Among  the  other  Volvocinae,  however,  a  few  or 
many  cells  live  together,  forming  a  small  colony — that  is, 

*  These  colonial  organisms,  the  Volvocinae,  are  the  objects  of  some 
contention  between  botanists  and  zoologists.  The  botanists  call  them 
plants  because  they  possess  a  cellulose  membrane  and  green  chroma- 
tophores,  and  exhibit  the  metabolism  characteristic  of  most  plants  ;  but 
most  zoologists  consider  them  to  be  animals  belonging  to  the  order 
Flagellata  of  the  Protozoa.  In  the  latest  authoritative  text-book  of 
zoo'logy,  that  of  Parker  and  Haswell  (1897),  they  are  so  classed. 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS       25 


there  is  formed  a  group  of  a  few  or  many  cells,  each  cell 
having  the  structure  of  the  simpler  unicellular  forms. 
These  cells  are  held  together  in  a  gelatinous  envelope,  and 
the  mass  is  usually  spherical  in  shape.  In  most  of  the 
colonies  each  of  the  cells  possesses  two  or  three  long,  pro- 
toplasmic, whiplash-like  hairs,  called  flagella,  and  by  the 
lashing -of  these  flagella  in  the  water  the  whole  group  swims 
about. 

14.  Gonium. — If,  when  one  of  the  simplest  animals  di- 
vided to  form  two  daughter  cells,  these  two  cells  did  not 
move  apart,  but  remained 
side  by  side  and  each  di- 
vided to  form  two  more, 
and  each  of  these  divided 
to  form  two  more,  and 
these  eight  divided  each 
into  two,  each  cell  com- 
plete and  independent  but 
all  remaining  together 
in  a  group  —  if  this  pro- 
cess should  take  place  we 
should  have  produced  a 
group  or  colony  of  sixteen 
cells,  each  cell  a  complete 
animal  capable  of  living 
independently  like  the 
other  simplest  animals, 
but  all  holding  together  B 

to  form  a  tiny,  flat,  plate-    FIG.  12.— Gonium  pectorale  (after  STEIN).    A, 

like  colony.  Now,  this  is  *j^^^  ab°ve;  B'  C°lony  8een 
precisely  what  takes  place 

in  the  case  of  those  colonial  Protozoa  belonging  to  the  genus 
Gonium  (Fig.  12).  When  the  mother  cell  of  Gonium  di- 
vides, the  daughter  cells  do  not  swim  apart,  but  remain 
side  by  side,  and  by  repeated  fission,  until  there  are  sixteen 
cells  side  by  side,  the  colony  is  formed.  Each  cell  of  the 


26 


ANIMAL  LIFE 


colony  eats  and  breathes  and  feels  for  itself  ;  each  can  and 
does  perform  all  the  processes  necessary  to  keep  it  alive. 
When  ready  to  multiply,  the  sixteen  cells  of  the  Gonium 
colony  separate,  and  each  cell  becomes  the  ancestor  of  a 
new  colony. 

15.  Pandorina. — Another  colony  usually  composed  of  six- 
teen cells  is  Pandorina,  but  the  cells  are  arranged  to  form 
a  spherical  instead  of  a  plate-like  colony  (Fig.  13).  In  Pan- 
dorina morum  the  colony  consists  of  sixteen  ovoid  cells  in 
a  spherical  jelly-like  mass.  Each  cell  has  two  flagella,  and 
by  the  lashing  of  all  the  flagella  the  whole  colony  moves 
through  the  water.  Food  is  taken  by  any  of  the  cells,  is 
assimilated,  and  the  cells  increase  in  size.  When  Pan- 
dorina is  ready  to  multiply,  each  cell  divides  repeatedly 
until  it  has  formed  sixteen  daughter  cells.  The  inclosing 
gelatinous  mass  which  holds  the  colony  together  dissolves, 

and  the  daughter  colonies  be- 
come free  and  swim  apart. 
Each  colony  soon  grows  to  the 
size  of  the  original  colony. 
This  kind  of  multiplication  or 
reproduction  may  be  continued 
for  several  generations.  But 
it  does  not  go  on  indefinitely. 
After  a  number  of  these  gener- 
ations has  been  produced  by 
simple  division,  the  cells  of  a 
colony  divide  each  into  eight 
instead  of  sixteen  daughter 
cells.  The  daughter  cells  are 
not  all  of  the  same  size,  but 
the  difference  is  hardly  notice- 
able. The  eight  cells  resulting  from  the  repeated  division 
of  one  of  the  original  cells  separate  and  swim  about  inde- 
pendently by  means  of  their  flagella.  If  one  of  these  cells 
comes  near  a  similar  free-swimming  cell  from  another 


PIG.  13.— Pandorina  sp.  (from  Na- 
ture). The  cells  composing  the 
colony  are  beginning  to  divide  to 
form  daughter  colonies. 


THE  LIFE   OF   THE  SLIGHTLY  COMPLEX  ANIMALS       27 


colony,  the  two  cells  conjugate  (Fig.  14) — that  is,  fuse  to 
form  a  single  cell.  This  new  cell  formed  by  the  fusion  of 
two,  develops  a  tough  enveloping  membrane  of  cellulose 
and  passes  into  what  is  called 
the  "resting  stage."  That  is, 
the  cell  remains  dormant  for  a 
shorter  or  longer  time.  It  may 
thus  tide  over  a  drought  or  a 
winter.  It  may  become  dry  or 
be  frozen,  yet  when  suitable 
conditions  of  moisture  or  tem- 
perature are  again  present  the 
outer  wall  breaks  and  the  pro- 
toplasm issues  as  a  large  free- 
swimming  cell,  which  soon  di- 
vides into  sixteen  daughter 
cells  which  constitute  a  new 
colony. 

16.  Eudorina.— Another  colo- 
nial protozoan  which  much  re- 
sembles Pandorina^  but  differs 
from  it  in  one  interesting  and 
suggestive  thing,  is  Eudorina. 
In  Eudorina  elegans  (Fig.  15) 
the  colony  is  spherical  and  is 
composed  of  sixteen  or  thirty- 
two  cells.  Each  of  these  cells 
can  become  the  parent  of  a  new 
colony  by  simple  repeated  divi- 
sion. But  this  simple  mode  of 
reproduction,  just  as  with  Pan- 
dorina, can  not  persist  indefi- 
nitely. There  must  be  conjuga- 
tion. But  the  process  of  mul- 
tiplication, which  includes  conjugation,  is  different  from 
that  process  in  Pandorina,  in  that  in  Eudorina  the  conju- 


B 


FIG.  14.  —  Pandorina  morum  (after 
GOEBEL).  Three  stages  in  the 
conjugation  and  formation  of  the 
resting  spore.  A,  two  cells  just 
fused;  B,  the  two  cells  completely 
fused,  but  with  flagella  still  per- 
sisting ;  C,  the  resting  spore. 


28 


ANIMAL  LIFE 


gating  cells  are  of  two  distinctly  different  kinds.  When 
this  kind  of  multiplication  is  to  take  place  in  the  case  of 
Eudorina  elegans,  to  choose  a  common  species,  some  of 
the  cells  of  a  colony  divide  into  sixteen  or  thirty -two 

minute  elongated  cells,  each 
provided  with  two  flagella. 
These  small  cells  escape 


FIG.  15.— Eudorina  elegans.    A,  a  mature  colony  (from  Nature);  B,  formation  of 
the  two  kinds  of  reproductive  cells. 

from  the  envelope  of  the  parent  cell,  remaining  for  some 
time  united  in  small  bundles.  Other  cells  of  the  colony 
do  not  divide,  but  increase  slightly  in  size  and  become 
spherical  in  shape.  When  a  bundle  of  the  small  cells 
comes  into  contact  with  some  of  these  large  spherical 
cells  the  bundle  breaks  up,  and  conjugation  takes  place 
between  the  small  flagellated  free-swimming  cells  and  the 
large  non-flagellate  spherical  cells.  Each  new  cell  formed 
by  the  fusion  of  one  of  the  small  and  one  of  the  large  cells 
develops  a  cellulose  wall  and  assumes  a  resting  stage. 
After  a  time  from  each  of  these  resting  spores  a  new  colony 
of  sixteen  or  thirty-two  cells  is  formed  by  direct,  repeated 
division. 

17.  Volvox. — Another  interesting  colonial  protozoan  is 
Volvox.     The  large  spherical  colonies  of    Volvox  globator 


THE  LIFE  OP  THE  SLIGHTLY  COMPLEX  ANIMALS       29 


(Fig.  16)  are  composed  of  several  thousand  cells,  arranged 
in  a   single  peripheral  layer  about  the  hollow  center  of 
the  ball.     The  cells  are  ovoid,  and  each  is  provided  with 
two  long  flagella  which  pro- 
ject out  into  the  water.  The 
lashing  of  the  thousands  of 
the   flagella  give  the  ball- 
like  colony  a  rotary  motion. 
The  cells  are  held  together 
by  a  jelly-like  intercellular 
substance  and  are  connect- 
ed with  each  other  by  fine 
protoplasmic  threads  which 
extend  from  the  body  pro- 
toplasm of   one  cell  to  the 
cells  surrounding  it.    When 
the  colony  is  full  grown  and 
ready   to    reproduce    itself          jj 
certain  cells  of  the  colony      ^&r     /^/    (f 
undergo      great      changes.          C     "  /'     *" 
Some  of  them   increase  in 
size  enormously,  having  re- 
serve food  material   stored 
in  them,  and  they  may  be 
called  the  egg  cells  of  the 
colony.    Eeproduction  may 
now  occur  by  simple  divi- 
sion of  one  of  these  great 
egg  cells  into  many  small 
cells,  all  held  together  in  a 

Common    envelope.        These    FIG.  16.— A,  Volvox  minor,   entire  colony 

form    a    daughter    colony 

which    escapes    from    the 

mother  colony  and  by  growth  and  further  division  comes  to 

be  a  new  full-sized  colony.     Or  reproduction  may  occur  in 

another,  more  complex,  way.     Certain  cells  of  the  colony 


B 


(from  Nature).   B,  C,  and  D,  reproductive 
cells  of  Volvox  globator. 


30  ANIMAL  LIFE 

divide  into  bundles  of  very  small,  slender  cells,  each  of 
which  is  provided  with  flagella.  The  remaining  cells  of 
the  colony  (that  is,  those  which  have  not  swollen  into  egg 
cells  or  divided  into  many — sixty-four  to  one  hundred  and 
twenty-eight — minute,  flagellate  cells)  remain  unchanged  for 
a  while  and  finally  die.  They  take  absolutely  no  part  in 
reproducing  the  colony.  One  of  the  minute  free-swim- 
ming cells  fuses  with  one  of  the  enormous  egg  cells,  the 
new  cell  thus  formed  being  a  resting  spore.  From  this 
resting  spore  a  new  colony  develops  by  repeated  division. 

18.  Steps  toward  complexity. — Within  the  group  of  Vol- 
vocince  there  are  plainly  several  steps  on  the  way  from 
simplicity  of  structure  to  complexity  of  structure.  Gonium, 
Pandorina,  Eudorina,  and  Volvox  form  a  series  proceeding 
from  the  simplest  animals  toward  the  complex  animals. 
In  Gonium  the  cells  composing  the  colony  are  all  alike  in 
structure,  and  each  one  is  capable  of  performing  all  the 
processes  or  functions  of  life.  In  Pandorina  and  Eudorina 
the  cells  are  at  first  alike,  but  there  is,  as  the  time  for 
reproduction  approaches,  a  differentiation  of  structure ; 
the  cells  of  the  colony,  all  of  which  take  part  in  the  process 
of  reproduction,  come  to  be  in  certain  generations  of  two 
kinds — an  inactive  large  kind  which  may  be  called  the  egg 
cells,  and  a  small,  active,  free-swimming  kind  which  seeks 
out  and  conjugates  with,  or,  we  may  say,  fertilizes  the  egg 
cells.  In  Volvox  there  is  a  new  differentiation.  Only  cer- 
tain particular  and  relatively  few  cells  take  part  in  repro- 
ducing the  colony;  most  of  the  cells  have  given  up  the 
power  or  function  of  reproduction.  These  cells,  when  the 
time  of  multiplication  comes,  simply  support  the  special 
reproductive  cells.  They  continue  to  waft  the  great  colony 
through  the  water  by  lashing  their  flagella ;  they  continue 
to  take  in  food  from  the  outside.  The  reproductive  cells 
devote  themselves  wholly  to  the  business  of  producing  new 
colonies,  of  perpetuating  the  species.  And  this  matter  of 
reproduction  is  less  simple  than  in  the  other  VolvocincB. 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS       31 

At  least  there  is  much  more  difference  between  the  two 
kinds  of  reproductive  cells.  The  egg  cells  are  compara- 
tively enormous,  and  they  are  stored  with  a  mass  of  food 
material.  The  fertilizing  cells  are  very  small,  but  very 
active  and  very  different  from  the  egg  cells.  We  have  in 
Volvox  the  beginnings  of  a  distinct  division  of  labor  and 
an  accompanying  differentiation  of  structure.  Certain 
cells  of  the  colony  do  certain  things,  and  are  modified  in 
structure  to  tit  them  specially  for  their  particular  duties. 
The  steps  from  the  simplest  structure  toward  a  complex 
structure  are  plainly  visible. 

19.  Individual  or  colony. — Is  the  Gonium  colony,  the 
Pandorina  colony,  or  the  Volvox  colony  a  group  of  several  or 
many  distinct  organisms,  or  is  it  to  be  considered  as  a  sin- 
gle organism  ?  With  Gonium,  which  we  may  call  the  sim- 
plest of  these  colonial  organisms,  the  colony  is  composed 
of  a  few  wholly  similar  cells  or  one-celled  animals,  each 
fully  capable  of  performing  all  the  life  processes,  each 
wholly  competent  to  lead  an  independent  life.  In  fact, 
each  does,  for  part  of  its  life,  live  independently,  as  we 
have  already  described.  In  the  case  of  Pandorina  and  Eu- 
dorina,  while  all  the  cells  are  for  most  of  the  lifetimo  of  the 
colony  alike  and  each  is  capable  of  living  independently, 
at  the  time  of  reproduction  the  cells  become  of  two  kinds. 
A  difference  of  structure  is  apparent,  and  for  the  perpetua- 
tion of  the  species  the  co-operation  of  these  different  kinds 
of  cells  is  necessary.  That  is,  it  is  impossible  for  a  single 
one  of  the  members  of  the  colony  to  reproduce  the  colony, 
except  for  a  limited  number  of  generations.  With  Volvox 
this  giving  up  of  independence  on  the  part  of  the  individual 
members  of  the  colony  is  more  marked.  There  is  a  real  in- 
terdependence among  the  thousands  of  cells  of  the  colony. 
The  function  of  reproduction  rests  with  a  few  particular 
cells,  and  for  the  perpetuation  of  the  species  there  is  demand- 
ed a  co-operation  of  two  distinct  kinds  of  reproductive  cells. 
The  great  majority  of  the  cells  take  no  part  in  reproduc- 


32  ANIMAL  LIFE 

tion.  They  can  perform  all  the  other  life  processes  ;  they 
move  the  colony  by  lashing  the  water  with  their  flagella ; 
they  take  in  food  and  assimilate  it ;  they  can  feel.  All  the 
cells  of  the  great  colony,  too,  are  intimately  connected  by 
means  of  protoplasmic  threads.  The  protoplasm  of  one 
cell  can  mingle  with  that  of  another  cell;  food  can  go 
from  cell  to  cell.  The  question  whether  the  Volvox  colony 
is  a  group  of  distinct  organisms  or  is  a  single  organism 
made  up  of  cells  among  which  there  is  a  simple  but  obvi- 
ous difference  in  structure  and  function  ;  in  other  words, 
whether  Volvox  is  a  colony  of  one-celled  animals,  of  Pro- 
tozoa, or  is  a  multicellular  animal,  one  of  the  Metazoa  (for 
so  all  the  many-celled  animals  are  called),  is  a  difficult  one 
to  decide.  Most  zoologists  class  the  Volvocinae  with  the 
Protozoa — that  is,  they  incline  to  consider  Gonium,  Pan- 
dorina,  Volvox,  and  the  other  Volvocinae  as  groups  or  col- 
onies of  one-celled  animals. 

20.  Sponges. — If  the  VolvocincB  be  considered  to  belong 
to  the  Protozoa,  the  sponges  are  the  simplest  of  all  the 
many-celled  animals.  Sponges  are  not  free-swimming  ani- 
mals, except  for  a  short  time  in  their  young  stage,  but  are 
fixed,  like  plants.  They  live  attached  to  some  solid  sub- 
stance on  the  sea  bottom.  They  resemble  plants,  too,  in 
the  way  in  which  the  body  is  modified  during  growth  by 
the  environment.  If  the  rock  to  which  the  young  sponge 
is  attached  is  rough  and  uneven,  the  body  of  the  sponge 
will  grow  go  as  to  fit  the  unevenness  ;  if  the  rock  surface  is 
smooth,  the  body  of  the  sponge  will  be  more  regular.  Thus 
a  sponge  may  be  said  to  have  no  fixed  shape  of  body  ;  indi- 
viduals of  the  same  species  of  sponge  differ  much  in  form. 
The  typical  form  of  the  sponges  is  that  of  a  short  cylinder 
or  vase  attached  by  one  end  and  with  the  upper  free  end 
open  (Fig.  17).  Many  individuals  of  one  kind  usually  live 
together  in  a  close  group  or  colony,  and  they  may  be  so 
attached  to  each  other  as  to  appear  like  a  branching  plant. 
This  branching  may  be  very  diffuse,  and  the  branches 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS       33 


may  become  so  interwoven  with  each  other  as  to  form  a 
very  complex  group.  A  sponge  is  composed  of  many  cells 
arranged  in  three  layers — that  is,  the  body  of  a  sponge  is  a 
cylinder  closed  at  one  end  whose  wall  is  composed  of  three 
layers  of  cells.  The  outer  layer  of 
cells  is  called  the  ectoderm^  and  the 
cells  composing  it  are  flat  and  are 
all  closely  attached  to  each  other. 
The  inner  layer  is  called  the  endo- 
derm,  and  its  cells  are  thicker  than 
those  of  the  ectoderm  ;  they  are 
also  closely  attached  to  each  other. 
Sometimes  they  are  provided  with 
flagella  like  the  flagellate  Protozoa. 
The  flagella  are,  however,  not  for  the 
purpose  of  locomotion,  but  for  creat- 
ing currents  in  the  water,  which 
bathes  the  interior  of  the  open  cylin- 
drical body.  The  middle  layer, 
called  the  mesoderm,  is  composed  of 
numerous  separate  cells  lying  in  a 
jelly-like  matrix.  From  these  meso- 
derm cells  fine  needles  or  spicules 
of  lime  or  silica  often  project  out 
through  the  ectoderm.  These  mi- 
nute sponge  spicules  are  of  a  great 
variety  of  shapes,  and  they  form  a 
sort  of  skeleton  for  the  support  of 
the  soft  body  mass.  All  over  the 
outer  surface  of  the  body  are  scat- 
tered fine  openings  or  pores,  which 
lead  through  the  walls  of  the  body 
into  the  inner  cavity.  This  cavity  is  of  course  also  con- 
nected with  the  outside  by  the  large  opening  at  the  free  or 
apical  end  of  the  body. 

There  is  hardly  any  differentiation  of  parts  among  the 
4 


FIG.  17.— One  of  the  simplest 
sponges,  Calcolynthus  pri- 
migeniw  (after  HAECKEL). 
A  part  of  the  outer  wall  is 
cut  away  to  show  the  in- 
side. 


ANIMAL  LIFE 


sponges.  As  in  the  Protozoa,  there  are  no  special  organs 
for  the  performance  of  special  functions.  The  sponge 
feeds  by  creating,  with  its  flagella,  water  currents  which 

flow  in  through  the  many  fine 
pores  of  the  body  and  out  from 
the  inner  body  cavity  through 
the  large  opening  at  the  free 
end  of  the  body.  These  cur- 
rents of  water  bear  fine  parti- 
cles of  organic  matter  which 
are  taken  up  by  the  cells  lining 
the  pores  and  body  cavity,  and 
assimilated.  There  are  no 
special  organs  of  digestion. 
Each  cell  takes  up  food  and 
digests  it.  The  water  cur- 
rents also  bring  air  to  these 
same  cells,  and  thus  the  sponge 
breathes.  Although  the 
sponge  as  a  whole  can  not 
move,  does  not  possess  the 
power  of  locomotion,  yet  the 
protoplasm  of  the  cells  has 
the  power  of  contracting,  just 
as  with  the  Protozoa,  and  the 
pores  can  be  opened  or  closed 
by  this  cellular  movement. 
Practically,  thus,  the  only 
FIG.  i8.-one  of  the  simple  sponges,  movements  the  sponge  can 

Prophysema      prtmordiale     (after 

HAECKEL).  The  body  is  represented  make  are  the  movements  made 
as  cut  in  two  longitudinally.    The  by  the  individual  cells. 

large  cells  of  the  inner  layer  are  the  ^  .. 

egg  ceils.  Jtteproauction     is     accom- 

plished by  a  process  of  divi- 
sion, or  by  a  process  of  conjugation  and  subsequent  division. 
In  its  simplest  way  multiplication  takes  place  by  a  group 
of  cells  separating  from  the  body  of  the  parent  sponge, 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS       35 

becoming  inclosed  in  a  common  capsular  envelope,  and  by 
repeated  division  and  consequent  increase  in  number  of 
cells  becoming  a  new  sponge.  This  is  reproduction  by 
"  budding."  The  "  buds,"  or  small  groups  of  cells  which 
separate  from  the  parent  sponge,  are  called  gemmules. 
Reproduction  in  the  more  complex  way  occurs  as  follows  : 
Some  of  the  free  amoeboid  cells  of  the  mesoderm  (the  mid- 
dle one  of  the  three  layers  of  the  body  wall)  become  en- 
larged and  spherical  in  form.  These  are  the  egg  cells. 
Other  mesodermic  cells  divide  into  many  small  cells,  which 
are  oval  with  a  long,  tapering,  tail-like  projection.  These 
cells  are  active,  being  able  to  swim  by  the  lashing  of  the 
tapering  tail.  These  are  the  fertilizing  cells.  The  two 
kinds  of  reproductive  cells  may  be  formed  in  one  sponge  ; 
if  so,  they  are  formed  at  different  times.  Or  one  sponge 
may  produce  only  egg  cells,  another  only  fertilizing  or, 
as  they  are  called,  sperm  cells.  Conjugation  takes  place 
between  a  sperm  cell  and  an  egg  cell.  That  is,  one  of  the 
small  active  sperm  cells  finds  one  of  the  large,  spherical, 
inactive  cells  and  penetrates  into  the  protoplasm  of  its 
bqjly.  The  two  cells  fuse  and  form  a  single  cell,  which 
may  be  called  the  fertilized  or  impregnated  egg.  This  fer- 
tilized egg,  remaining  in  the  body  mass  of  the  parent 
sponge,  divides  repeatedly,  the  new  cells  formed  by  this 
division  remaining  together.  The  young  or  embryo  sponge 
finally  escapes  from  the  body  of  the  parent  sponge,  and 
lives  for  a  short  time  as  an  active  free-swimming  animal. 
Its  body  consists  of  an  oval  mass  of  cells,  of  which  those  on 
one  side  are  provided  with  cilia  or  swimming  hairs.  The 
cells  of  the  body  continue  to  divide  and  to  grow,  and  the 
body  shape  gradually  changes.  The  young  sponge  finally 
becomes  attached  to  some  rock,  the  body  assumes  the  typi- 
cal cylindrical  shape,  an  aperture  appears  at  the  free  end, 
and  small  perforations  appear  on  the  surface.  The  sponge 
becomes  full  grown. 

Those  of  us  who  do  not  live  in  the  vicinity  of  the  sea- 


36  ANIMAL  LIFE 

shore  where  sponges  are  found  can  not  observe  the  struc- 
ture and  life  history  of  living  specimens.  There  are,  how- 
ever, among  the  thousand  and  more  kinds  of  sponges  a  few 
kinds  that  live  in  fresh  water,  and  these  are  so  widely 
spread  over  the  earth  that  examples  of  them  can  be  found 
in  almost  any  region.  They  belong  to  the  genus  Spongilla, 
and  thirty  or  more  species  or  kinds  of  Spongilla  are  known. 
In  standing  or  slowly  flowing  water,  Spongilla  grows  erect 
and  branching,  like  a  shrub  or  miniature  tree ;  in  swift 
water  it  grows  low  and  spreading,  forming  a  sort  of  mat 
over  the  surface  to  which  it  is  attached.  Reproduction 
takes  place  very  actively  by  the  process  of  budding.  The 
budded-off  gemmules  are  spherical  in  shape,  and  the  cells 
of  each  gemmule  are  inclosed  in  an  envelope  composed  of 
siliceous  spicules  of  peculiar  shape.  These  gemmules  are 
formed  in  the  body  substance  of  the  parent  sponge  toward 
the  end  of  the  year,  and  are  set  free  by  the  decaying  of 
that  part  of  the  body  of  the  parent  sponge  in  which  they 
lie.  They  sink  to  the  bottom  of  the  pond  or  brook,  and 
lie  there  dormant  until  the  following  spring.  Then  they 
develop  rapidly  by  repeated  division  of  the  cells  and 
growth. 

It  is  not  the  purpose  here  to  describe  the  many  and 
interesting  kinds  of  sponges  which  inhabit  the  ocean.  The 
sponge  of  the  bathroom  is  simply  the  skeleton  of  a  large 
sponge  or  group  of  sponges.  The  skeleton  here  is  not 
composed  of  lime  or  silica,  but  of  a  tough,  horny  substance, 
which  is  secreted  by  cells  of  the  mesodermal  layer  of  the 
body  wall  of  the  sponge.  This  substance  is  called  spongin, 
and  is  a  substance  allied  to  silk  in  its  chemical  composi- 
tion. All  the  commercial  sponges,  the  spongin  skeletons, 
belong  to  one  genus — Spongia.  These  sponges  grow  espe- 
cially abundantly  in  the  Mediterranean  and  Red  Seas,  and 
in  the  Atlantic  Ocean  off  the  Florida  reefs,  and  on  the 
shores  of  the  Bahama  Islands.  The  sponges  are  pulled 
up  by  divers,  or  by  means  of  hooks  or  dredges.  The 


THE  LIFE  OP  THE  SLIGHTLY  COMPLEX  ANIMALS       37 

living  matter  soon  dies  and  decays,  leaving  the  horny 
skeleton,  which  when  cleaned  and  trimmed  is  ready  for 
use. 

The  most  beautiful  sponges  are  those  with  siliceous 
skeletons.  The  fine  needles  or  threads  of  glass,  arranged 
often  in  delicate  and  intricate  pattern,  make  these  sponges 
objects  of  real  beauty. 

21.  Polyps,  corals,  and  jelly-fishes. — The  general  or  typ- 
ical plan  of  body  structure  of  those  animals  which  come 
next  in  degree  of  complexity  to  the  sponges  can  be  best 
understood  by  imagining  the  typical  cylindrical  body  of  a 
sponge  modified  in  the  following  way:   The    middle  one 
of  the  three  layers  of  the  body  wall  not  to  be  composed 
of  cells  in  a  gelatinous  mass,  but  to  be  simply  a  thin  non- 
cellular    membrane ;   the  body  wall  to  be  pierced  by  no 
fine  openings  or  pores,  so  that  tthe  interior  cavity  of  the 
body  is   connected   with  the  outside  only  by  the   single 
large  opening  at  the  free  end,  and  this  opening  to  be  sur- 
rounded by  a  circlet  of  arm-like  processes  or   tentacles, 
continuations   of  the  body  wall  and  similarly  composed. 
Such  a  body  structure  is  the  general  or  fundamental  one 
for  all  polyps,  corals,  sea-anemones,  and  jelly-fishes.     The 
variety  in  shape  and  the  superficial  modifications  of  this 
type-plan  are  many  and  striking ;  but,  after  all,  the  type- 
plan  is  recognizable  throughout  the  whole  of  this  great 
group  of  animals.     Perhaps  the  simplest  representative  of 
the  group  is  a  tiny  polyp  which  grows  abundantly  in  the 
fresh-water  streams  and  pools,  and  can  be  readily  obtained 
for  observation.     It  is  called  Hydra. 

22.  Hydra,— The  body  of   Hydra  (Fig.   19),  which  is 
very  small  and  appears  to  the  unaided  eye  as  a  tiny  white 
or  greenish  gelatinous  particle  attached  to  some  submerged 
stone,  bit  of  wood,  or  aquatic  plant,  is  a  simple  cylinder 
attached  by  one  end  to  the  stone  or  weed.     The  other  free 
end  is  contracted  so  as  to  be  conical,  and  it  is  narrowly 
open.     Around  the  opening  are  six  or  eight  small  waving 

4 


38 


ANIMAL  LIFE 


tentacles.  The  wall  of  the  cylinder  is  composed  of  an 
outer  and  an  inner  layer  of  cells  and  a  thin  non-cellular 
membranous  layer  between  them.  The  tentacles  are  hol- 
low and  are  simple  extensions  of  the  body  wall.  The  cells 
of  the  outer  layer,  or  ectoderm,  are  not  all  alike.  Some 
are  smaller  than  the  others  and  appear  to  be  crowded  in 


FIG.  19.— The  fresh-water  polyp,  Hydra  vulgaris.  A,  in  expanded  condition,  and 
in  contracted  condition;  B,  cross  section  of  body,  showing  the  two  layers  of 
cells  which  make  up  the  body  wall. 

between  the  bases  or  inner  ends  of  the  larger  ones.  The 
inner  ends  of  the  large  cells  are  extended  as  narrow-pointed 
prolongations  directed  at  right  angles  with  the  rest  of  the 
cell.  These  processes  are  very  contractile  and  are  called 
muscle  processes.  Each  one  is  simply  a  continuation  of 
the  protoplasm  of  the  cell  body,  which  is  especially  con- 
tractile. Some  of  the  smaller  ectoderm  cells  are  very 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS       39 

irregular  in  shape  and  possess  specially  large  nuclei.  These 
cells  are  more  irritable  or  sensitive  than  the  others  and 
are  called  nerve  cells.  The  ectoderm  cells  of  the  base  or 
foot  of  the  Hydra  are  peculiarly  granular,  and  secrete  a 
sticky  substance  by  which  the  Hydra  holds  fast  to  the 
stone  or  weed  on  which  it  is  found.  These  cells  are  called 
gland  cells.  Imbedded  in  many  of  the  larger  ectoderm 
cells,  especially  those  of  the  tentacles,  are  small  oval  sacs, 
in  each  of  which  lies  folded  or  coiled  a  fine  long  thread. 
When  the  tentacles  touch  one  of  the  small  animals  which 
serve  Hydra  as  food,  these  fine  threads  shoot  out  from 
their  sacs  and  so  poison  or  sting  the  prey  that  it  is 
paralyzed.  The  tentacles  then  contract  and  bend  inward, 
forcing  the  captured  animal  into  the  mouth  opening 
in  the  center  of  the  circle  of  tentacles.  Through  the 
mouth  opening  the  prey  enters  the  body  cavity  of  Hydra 
and  is  digested  by  the  cells  lining  this  cavity.  These  cells 
belonging  to  the  inner  layer  of  the  body  wall  or  endoderm 
are  mostly  large,  and  each  contains  one  or  more  contractile 
vacuoles.  From  the  free  ends — the  ends  which  are  next  to 
the  body  cavity — of  these  cells  project  pseudopods  or  fine 
flagella.  These  projections  are  constantly  changing :  now 
two  or  three  short,  blunt  pseudopods  are  projecting  into 
the  body  cavity ;  now  they  are  withdrawn,  and  a  few  fine, 
long  flagella  are  projected.  In  addition  to  these  cells  there 
are  in  the  endoderm,  especially  abundant  near  the  mouth 
opening  and  wholly  lacking  in  the  tentacles  and  at  the 
base  of  the  body,  many  long,  narrow,  granular  cells.  They 
are  gland  cells  which  secrete  a  digestive  fluid.  The  food 
captured  by  the  tentacles  and  taken  in  through  the 
mouth  opening  disintegrates  in  the  body  cavity,  or  diges- 
tive cavity,  as  it  may  be  called.  The  digestive  fluid  se- 
creted by  the  gland  cells  of  the  endoderm  acts  upon  it, 
so  that  it  becomes  broken  into  small  parts.  These  par- 
ticles are  probably  seized  by  the  pseudopods  of  the  other 
endoderm  cells  and  are  taken  into  the  body  protoplasm 


40  ANIMAL  LIFE 

of  these  cells.  The  ectoderm  cells  do  not  take  food 
directly,  but  receive  nourishment  only  through  the  endo- 
derm  cells. 

Hydra  is  not  permanently  attached.  It  holds  firmly 
to  the  submerged  stone  or  weed  by  means  of  the  sticky 
secretion  from  the  ectodermal  gland  cells  of  its  base,  but  it 
can  loosen  itself,  and  by  a  slow  creeping  or  gliding  move 
along  the  surface  of  the  stone  to  another  spot.  Even  when 
attached,  the  form  of  the  body  changes ;  it  extends  itself 
longitudinally,  or  it  contracts  into  a  compact  globular  mass. 
The  tentacles  move  about  in  the  water,  and  are  continually 
contracting  or  extending. 

Like  Volvox  and  the  sponges,  those  other  slightly  com- 
plex animals  we  have  already  considered,  Hydra  has  two 
methods  of  multiplication.  In  the  simpler  way,  there 
appears  on  the  outer  surface  of  the  body  a  little  bud  which 
is  composed,  at  first,  of  ectoderm  cells  alone ;  but  soon  it  is 
evident  that  it  is  a  budding,  or  outpushing,  of  the  whole 
body  wall,  ectoderm,  endoderm,  and  middle  membrane.  In 
a  few  hours  the  bud  has  six  or  eight  tiny,  blunt  tentacles, 
a  mouth  opening  appears  at  the  free  end,  and  the  little 
Hydra  breaks  off  from  the  parent  body  and  leads  an  inde- 
pendent existence.  In  the  more  complex  way,  two  kinds  of 
special  reproductive  cells  are  produced  by  each  individual, 
viz.,  large,  inactive,  spherical  egg  cells,  and  small,  active 
sperm  cells,  each  with  an  oval  part  or  head  (consisting  of 
the  nucleus)  and  a  slender,  tapering  tail-like  part  (consist- 
ing of  the  cytoplasm).  The  egg  cell  lies  inclosed  in  a  layer 
of  thin,  surrounding  cells,  which  compose  a  capsule  for  it. 
When  the  egg  cell  is  ready  for  fertilization  this  capsule 
breaks,  and  one  of  the  active  sperm  cells  finds  its  way  to 
and  fuses  with  the  egg  cell.  The  fertilized  egg  cell  now 
divides  into  several  cells,  which  remain  together.  The 
outer  ones  form  a  hard  capsule,  and  thus  protected  the 
embryo  falls  to  the  bottom,  and  after  lying  dormant  for 
awhile  develops  into  a  Hydra. 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS      41 

23.  Differentiation  of  the  body  cells. — In  Hydra  we  have 
the  beginnings  of  complexity  of  structure  carried  a  step 
further  than  in  the  sponges.     The  division  of  labor  among 
the  cells  composing  the  body  is  more  pronounced,  and  the 
structural  modification  of  the  different  cells  to  enable  them 
better  to  perform  their  special  duties  is  obvious.     Some  of 
the  cells  of  the  body  specially  devote  themselves  to  food- 
taking  ;  some  specially  to  the  digestion  of  the  food ;  some 
are  specially  contractile,  and  on  them  the  movements  of 
the  body  depend,  while   others  are   specially  irritable  or 
sensitive,  and  on  them  the  body  depends  for  knowledge  of 
the  contact  of  prey  or  enemies.     In  the  lasso  cells — those 
with  the  stinging  threads — there  is  a  very  wide  departure 
from  the  simple  primitive  type  of  cells.    There  is  in  Hydra 
a  manifest  differentiation  of  the  cells  into  various  kinds  of 
cells.     The  beginnings  of  distinct  tissues  and  organs  are 
foreshadowed. 

The  individuals  of  Hydra  live,  usually,  distinct  from 
each  other.  There  is  no  tree-like  colony,  as  with  the  sponges. 
But  most  of  the  other  polyps  do  live  in  this  colonial  manner. 
The  new  polyps  which  develop  as  buds  from  the  body  of 
the  parent  do  not  separate  from  the  parent,  but  remain 
attached  by  their  bases.  They,  in  turn,  produce  new 
polyps  which  remain  attached,  so  that  in  time  a  branching, 
tree-like  colony  is  formed. 

24.  Medusae  or  jelly-fishes. — Most    of  the   other  polyps 
differ  from  Hydra  also  in  producing,  in  addition  to  ordi- 
nary polyp  buds,  buds  which  develop  into  bell-shaped  struc- 
tures called  medusae  (Fig.  20).     These  medusae  consist  of  a 
soft  gelatinous  bell-  or  umbrella-shaped  body,  with  a  short 
clapper   or  stem  which  has  an  opening  at  its  free  end. 
From  the  edge  of  the  bell  or  umbrella  four  pairs  of  tenta- 
cles project.     The  medusae  usually  separate  from  the  parent 
polyp  and  live  an  independent,  free-swimming  life.     These 
are  the  beautiful  animals  commonly  known  as  jelly-fishes. 
The  medusae  or  jelly-fishes  produce  special  reproductive 


42  ANIMAL  LIFE 

cells,  a  single  medusa  producing  only  one  kind  of  such  cells 
— that  is,  producing  either  egg  cells  alone  or  sperm  cells 
alone.  The  active  sperm  cells  produced  by  one  medusa 
find  their  way  to  an  egg  cell  producing  medusa,  and  fuse 
with  or  fertilize  these  egg  cells.  The 
fertilized  egg  develops  into  a  small, 
oval,  free-swimming  embryo  called  a 
planula,  which  finally  attaches  itself 
to  a  stone  or  bit  of  wood  or  seaweed, 
and  grows  to  be  a  simple  cylindrical 
polyp  attached  at  its  base  and  with 
mouth  and  tentacles  at  its  free  end. 
This  polyp  gives  rise  by  budding  to 
new  polyps,  which  remain  attached 
to  it,  and  gradually  a  new  tree-like 
colony  is  formed.  From  this  polyp 
or  this  colony  new  medusae  bud  off, 
swim  away,  and  finally  produce  new 

*-Z£%2££**  Polyps-  Th™ there  is  in  the  lile  of 

the  polyps  what  is  called  an  alterna- 
tion of  generations.  There  are  two  kinds  of  individuals 
which  evidently  belong  to  the  same  species '  of  animal,  or, 
put  in  another  way,  one  kind  of  animal  has  two  distinct 
forms.  This  appearance  of  one  kind  of  animal  in  two 
forms  is  called  dimorphism.  We  shall  see  later,  that  one 
kind  of  animal  may  appear  in  more  than  two  forms  ;  such 
a  condition  is  called  polymorphism.  In  alternation  of  gen- 
erations we  have  the  polyp  animal  appearing  in  one  genera- 
tion as  a  fixed  cylindrical  polyp,  while  in  the  next  generation 
it  is  a  free-swimming,  umbrella-shaped  medusa  or  jelly-fish. 
The  polyps  which  are  dimorphic — that  is,  have  a  polyp 
form  of  individual  and  a  medusa  form  of  individual — show 
more  differentiation  in  structure  than  the  simple  Hydra. 
This  further  differentiation  is  especially  apparent  in  the 
medusae  or  jelly-fishes.  Here  the  nerve  cells  are  aggregated 
in  little  groups  arranged  along  the  edge  of  the  umbrella 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS      43 

to  form  distinct  sense  organs.  The  muscle  processes  are 
better  developed,  and  the  digestive  cavity  is  differentiated 
into  central  and  peripheral  portions.  In  these  dimorphic 
polyps  the  fixed  polyp  individuals  reproduce  by  the  simple 
way  of  budding,  while  the  medusa  individuals  reproduce 
by  producing  special  reproductive  cells  of  two  kinds,  which 
must  fuse  to  form  a  cell  capable  of  developing  into  a  new 
polyp. 

25.  Corals. — There  are  many  kinds  of  polyps  and  jelly- 
fishes,  and  they  present  a  great  variety  of  shape  and  size 
and  general  appearance.  Many  polyps  exist  only  in  the 
true  polyp  form,  never  producing  medusae.  Others  have 


FIG.  21.— A  polyp,  or  sea-anemone  (Metridium  dianthus). 

only  the  medusa  form.  Some  live  in  colonies,  and  others 
are  always  solitary.  The  animals  we  know  as  corals  are 
polyps  which  live  in  enormous  colonies,  and  which  exist 
only  in  the  true  polyp  form,  not  producing  medusae.  They 


Fro.  22.— Coral  island  (Nanuku  Levu,  of  the  Fiji  group).    (After  a  photograph 
by  MAX  AGASSIZ.) 


FIG.  23.— Shore  of  a  coral  island,  with  cocoanut  palms.    /After  a  photograph.) 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS      45 

form  a  firm  skeleton  of  lime  (calcium  carbonate),  and  after 
their  death  these  skeletons  persist,  and  because  of  their 
abundance  and  close  massing  form  great  reefs  or  banks  and 
islands.  Coral  islands  occur  only  in  the  warmer  oceans. 
In  the  Atlantic  they  are  found  along  the  coasts  of  southern 
Florida,  Brazil,  and  the  West  Indies ;  in  the  Pacific  and 
Indian  Oceans  there  are  great  coral  reefs  on  the  coast  of 
Australia,  Madagascar,  and  elsewhere,  and  certain  large 


FIG.  24. — Organ-pipe  coral. 

groups  of  inhabited  islands  like  the  Fiji,  Society,  and 
Friendly  Islands  are  composed  exclusively  of  coral  islands. 
More  than  two  thousand  kinds  of  living  corals  are  known, 
and  their  skeletons  offer  much  variety  in  structure  and 
appearance.  Brain  coral,  organ-pipe  coral  (Fig.  24),  the 
well-known  red  coral  from  Italy  and  Sicily,  used  as  jewelry, 
and  the  sea  pens  and  sea  fans  are  among  the  better  known 
and  more  beautiful  kinds  of  coral  skeletons. 

26.  Colonial  jelly-fishes. — While  many  of  the  medusae  or 
jelly-fishes  are  another  form  of  individual  of  a  true  fixed 
polyp,  many  of  the  larger  and  more  beautiful  jelly-fishes  do 
not  exist  in  any  other  form.  Some  of  these  larger  jelly- 
fishes  are  several  feet  in  diameter,  and  when  cast  up  on  the 
beach  form  a  great  shapeless  mass  of  soft,  jelly-like  sub- 


46  ANIMAL  LIFE 

stance.  The  bodies  of  all  jelly-fishes  are  soft  and  gelatinous, 
the  body  substance  containing  hardly  one  per  cent  of  solid 
matter.  It  is  mostly  water.  Many  jelly-fishes  are  beauti- 
fully and  strikingly  colored,  and  as  they  swim  slowly  about 
near  the  surface  of  the  ocean,  lazily  opening  and  shutting 
their  iridescent,  umbrella-like  bodies,  they  are  among  the 
most  beautiful  of  marine  organisms.  When  one  of  the 
jelly-fishes  is  taken  from  the  water,  however,  it  quickly  loses 
its  brilliant  colors,  and  dries  away  to  a  shapeless,  shrivel- 
ing, sticky  mass. 

Some  of  the  most  beautiful  of  the  jelly-fishes  belong 
to  a  group  called  the  Siphonophora.  These  jelly-fishes  are 
elongate  and  tube-like  rather  than  umbrella-  or  bell-shaped, 
and  they  are  polymorphic — that  is,  there  are  several  dif- 
ferent forms  of  individuals  belonging  to  a  single  kind 
or  species.  The  Siphonophora  are  all  free-swimming,  but 
nevertheless  form  small  colonies.  In  the  Mediterranean 
Sea  and  in  other  southern  ocean  waters  the  surface  may  be 
covered  for  great  areas  by  these  brilliantly  colored  jelly-fish 
colonies,  each  of  which  looks,  as  a  celebrated  German  natu- 
ralist has  said,  like  a  swimming  flower  cluster  whose  parts, 
flowers,  stems,  and  leaves  seem  to  be  made  of  transparent 
crystal,  but  which  possess  the  life  and  soul  of  an  animal. 
An  abundant  species  of  these  Siphonophora  (Fig.  25)  is  com- 
posed of  a  slender,  flexible,  floating,  central  stem  several  feet 
long,  to  which  are  attached  thousands  of  medusa  and  polyp 
individuals  representing  several  different  kinds  of  forms, 
each  kind  of  individual  being  specially  modified  or  adapted 
to  perform  some  one  duty.  The  central  stem  is  a  greatly 
elongated  polyp  individual,  whose  upper  end  is  dilated  and 
filled  with  air  to  form  a  float.  This  individual  holds  up 
the  whole  colony.  Grouped  around  this  central  stem  just 
below  the  float  are  many  bell-shaped  bodies  which  alter- 
nately open  and  close,  and  by  thus  drawing  in  and  expelling 
water  from  their  cavities  impel  the  whole  colony  through 
the  water.  These  bell-shaped  structures  are  attached  me- 


THE  LIFE  OF  THE  SLIGHTLY  COMPLEX  ANIMALS       47 


dusa  individuals,  whose 
business  it  is  to  be  the 
locomotive  organs  for  the 
colony.  These  medusae 
are  without  tentacles,  and 
take  no  food  and  produce 
no  young.  They  have 
given  up  the  power  of 
performing  these  other 
life  processes,  and  devote 
themselves  wholly  to  the 
business  of  locomotion. 
From  the  lower  end  of  the 
central  stem  rises  a  host  of 
structures,  among  which 
several  distinct  kinds  are 
readily  perceived.  One 
kind  is  composed  of  a  pear- 
shaped  hollow  body  open 
at  its  free  end,  and  bear- 
ing a  long  tentacle  which 
is  furnished  with  numer- 
ous groups  of  stinging 
cells.  These  are  the  polyp 
individuals  whose  especial 
business  it  is  to  capture 
and  sting  prey  and  to  eat 
it.  -  These  individuals  are 
the  food -getters  for  the 
colony.  Scattered  among 
these  stinging,  feeding 
polyps,  are  numerous 
smaller  individuals  with 
oval,  closed  body,  each 
bearing  a  long,  slender 
thread.  These  threads 


FIG.  25.— A  colonial  jelly-fish,  Physoph&ra 
(after  HAECKEL).  At  the  top  is  the  float 
polyp,  around  its  stem  the  swimming 
medusae,  and  below  are  the  feeding,  feel- 
ing, protecting,  and  reproducing  polyps 
and  medusa. 


48  ANIMAL  LIFE 

are  very  sensitive,  and  the  polyps  bearing  them  have  for 
special  function  that  of  feeling  or  being  sensible  of  stimuli 
from  without.  They  are  the  sense  organs  or  sense  indi- 
viduals of  the  colony.  Finally,  there  are  two  other  kinds 
of  structures,  or  individuals  which  produce  the  special 
reproductive  cells  for  the  perpetuation  of  the  species. 
These  are  the  modified  medusa  individuals,  and  one  kind, 
larger  than  the  other,  produces  the  active  sperm  cells, 
while  the  other  produces  the  inactive  egg  cells. 

27.  Increase  in  the  degree  of  complexity. — In  the  corals, 
sea-anemones,  and  jelly-fishes  there  is  plainly  much  more 
of  a  division  of  labor  among  the  various  parts  of  an  indi- 
vidual and  much  more  modification  of  these  parts— that  is, 
much  more  structural  complexity  than  among  the  sponges 
and  Hydra.  And  these,  in  their  turn,  are  more  complex  than 
are  the  colonial  Protozoa,  the  Volvocinae.  There  is  a  great 
difference  in  degree  of  complexity  among  the  slightly  com- 
plex animals.  But  the  various  groups  of  these  animals 
which  we  have  studied  can  all  be  arranged  roughly  in  a 
series  beginning  with  the  least  complex  among  them  and 
ascending  to  the  most  complex.  And  in  this  series,  and 
in  the  always  accompanying  division  of  labor  among  the 
different  parts,  the  gradual  increase  in  complexity  is  beau- 
tifully shown. 

From  an  animal  composed  of  many  structurally  simi- 
lar cells,  each  cell  capable  of  performing  all  the  life  pro- 
cesses, we  pass  to  an  animal  composed  of  cells  of  a  few 
different  kinds,  of  slight  structural  diversity.  Each  kind 
of  cell  devotes  itself  especially  to  a  certain  few  life  pro- 
cesses or  functions.  Next  we  find  an  animal  in  which  the 
cells  of  one  kind  are  specially  aggregated  to  form  a  single 
part  of  the  body  which  is  specially  devoted  to  the  perform- 
ance of  a  single  function.  This  diversity  among  the  cells 
increases,  this  aggregation  of  similar  cells  to  form  special 
parts  or  organs  increases,  and  the  division  of  labor  or 
assignment  of  special  functions  to  special  organs  becomes 


THE  LIFE  OP  THE  SLIGHTLY  COMPLEX  ANIMALS       49 

more  and  more  pronounced.  Among  the  more  complex 
polyps  and  jelly-fishes  the  contractile  cells  form  distinct 
muscle  fibers  and  muscles ;  the  sensitive  cells  form  dis- 
tinct nerve  cells  and  nerve  fibers  which  are  arranged  in  a 
primitive  nervous  system ;  the  digestive  cavity  becomes 
complex  and  composed  of  different  portions  ;  the  reproduc- 
tive cells  are  formed  by  special  organs,  and  the  distinction 
between  the  egg  cells  and  the  sperm  cells — that  is,  be- 
tween the  female  reproductive  elements  and  the  male 
reproductive  elements — becomes  more  pronounced. 

We  have  followed  this  increase  or  development  of  struc- 
tural and  physiological  complexity  from  simplest  animals 
to  fairly  complex  ones.  The  principle  of  this  development 
of  complexity  is  evident.  It  will  not  be  profitable  to  at- 
tempt to  follow  in  detail  this  development  among  the 
higher  animals.  The  complex  animals  are  complex  be- 
cause their  life  processes  are  performed  by  special  parts  of 
their  body,  which  parts  are  specially  modified  so  as  to  perform 
these  processes  well.  The  animals  which  are  more  complex 
than  those  we  have  studied  differ  from  these  simply  in  the 
degree  of  complexity  attained.  In  order  to  understand 
this  better  we  shall  not  further  consider  special  groups  of 
animals,  but  special  processes  or  functions,  and  attempt  to 
see  how  the  modification  and  increase  in  complexity  of 
structure  goes  hand  in  hand  with  the  increase  of  elaborate- 
ness or  complexity  in  the  performance  of  function. 


CHAPTEE  III 

THE   MULTIPLICATION  OF   ANIMALS   AND    SEX 

28.  All  life  from  life. — On  the  performance  of  the  func- 
tion of  reproduction  or  multiplication  depends  the  exist- 
ence or  perpetuation  of  the  species.  Although  an  animal 
may  take  food  and  perform  all  the  functions  necessary  to 
its  own  life,  it  does  not  fulfill  the  demands  of  successful 
existence  unless  it  reproduces  itself.  Some  individuals  of 
every  species  must  produce  offspring  or  the  species  becomes 
extinct.  We  have  seen  in  our  study  of  the  simple  animals 
that  the  function  of  reproduction  is  the  first  function  to 
become  differentiated  in  the  ascent  from  simplest  animals 
to  complex  animals.  The  first  division  of  labor  among  the 
cells  composing  the  bodies  of  the  slightly  complex  animals 
«,nd  the  first  structural  differences  among  the  cells  are 
connected  with  the  performance  of  the  function  of  repro- 
duction or  multiplication. 

We  are  all  so  familiar  with  the  fact  that  a  kitten 
comes  into  the  world  only  through  being  born,  as  the  off- 
spring of  parents  of  its  kind,  that  we  shall  likely  not  appre- 
ciate at  first  the  full  significance  of  the  statement  that  all 
life  comes  from  life ;  that  all  organisms  are  produced  by 
other  organisms.  Nor  shall  we  at  first  appreciate  the  im- 
portance of  the  statement.  This  is  a  generalization  of 
modern  times.  It  has  always  been  easy  to  see  that  cats 
and  horses  and  chickens  and  the  other  animals  we  famil- 
iarly know  give  birth  to  young  or  new  animals  of  their 
own  kind ;  or,  put  conversely,  that  young  or  new  cats  and 
horses  and  chickens  come  into  existence  only  as  the  off- 
50 


THE  MULTIPLICATION  OF  ANIMALS  AND  SEX       51 

spring  of  parents  of  their  kind.  And  in  these  latter  days  of 
microscopes  and  mechanical  aids  to  observation  it  is  even 
easy  to  see  that  the  smaller  animals,  the  microscopic  organ- 
isms, come  into  existence  only  as  they  are  produced  by  the 
division  of  other  similar  animals,  which  we  may  call  their 
parents.  But  in  the  days  of  the  earlier  naturalists  the 
life  of  the  microscopic  organisms,  and  even  that  of  many 
of  the  larger  but  unfamiliar  animals,  was  shrouded  in 
mystery.  And  what  seem  to  us  ridiculous  beliefs  were 
held  regarding  the  origin  of  new  individuals. 

29.  Spontaneous  generation. — The  ancients  believed  that 
many  animals  were  spontaneously  generated.  The  early 
naturalists  thought  that  flies  arose  by  spontaneous  genera- 
tion from  the  decaying  matter  of  dead  animals ;  from  a 
dead  horse  come  myriads  of  maggots  which  change  into 
flesh  flies.  Frogs  and  many  insects  were  thought  to  be 
generated  spontaneously  from  mud.  Eels  were  thought  to 
arise  from  the  slime  rubbed  from  the  skin  of  fishes.  Aris- 
totle, the  Greek  philosopher,  who  was  the  greatest  of  the 
ancient  naturalists,  expresses  these  beliefs  in  his  books.  It 
was  not  until  the  middle  of  the  seventeenth  century- 
Aristotle  lived  three  hundred  and  fifty  years  before  the 
birth  of  Christ — that  these  beliefs  were  attacked  and  be- 
gan to  be  given  up.  In  the  beginning  of  the  seventeenth 
century  William  Harvey,  an  English  naturalist,  declared 
that  every  animal  comes  from  an  egg,  but  he  said  that  the 
egg  might  "  proceed  from  parents  or  arise  spontaneously  or 
out  of  putrefaction."  In  the  middle  of  the  same  century 
Eedi  proved  that  the  maggots  in  decaying  meat  which  pro- 
duce the  flesh  flies  develop  from  eggs  laid  on  the  meat  by 
flies  of  the  same  kind.  Other  zoologists  of  this  time  were 
active  in  investigating  the  origin  of  new  individuals.  And 
all  their  discoveries  tended  to  weaken  the  belief  in  the 
theory  of  spontaneous  generation. 

Finally,  the  adherents  of  this  theory  were  forced  to 
restrict  their  belief  in  spontaneous  generation  to  the  case 


52  ANIMAL  LIFE 

of  a  few  kinds  of  animals,  like  parasites  and  the  animalcules 
of  stagnant  water.  It  was  maintained  that  parasites  arose 
spontaneously  from  the  matter  of  the  living  animal  in 
which  they  lay.  Many  parasites  have  so  complicated  and 
extraordinary  a  life  history  that  it  was  only  after  long  and 
careful  study  that  the  truth  regarding  their  origin  was  dis- 
covered. But  in  the  case  of  every  parasite  whose  life  his- 
tory is  known  the  young  are  offspring  of  parents,  of  other 
individuals  of  their  kind.  No  case  of  spontaneous  genera- 
tion among  parasites  is  known.  The  same  is  true  of  the 
animalcules  of  stagnant  water.  If  some  water  in  which 
there  are  apparently  no  living  organisms,  however  minute, 
be  allowed  to  stand  for  a  few  days,  it  will  come  to  be 
swarming  with  microscopic  plants  and  animals.  Any  or- 
ganic liquid,  as  a  broth  or  a  vegetable  infusion  exposed  for 
a  short  time,  becomes  foul  through  the  presence  of  innumer- 
able bacteria,  infusoria,  and  other  one-celled  animals  and 
plants,  or  rather  through  the  changes  produced  by  their 
life  processes.  But  it  has  been  certainly  proved  that  these 
organisms  are  not  spontaneously  produced  by  the  water  or 
organic  liquid.  A  few  of  them  enter  the  water  from  the 
air,  in  which  there  are  always  greater  or  less  numbers  of 
spores  of  microscopic  organisms.  These  spores  (embryo  or- 
ganisms in  the  resting  stage)  germinate  quickly  when  they 
fall  into  water  or  some  organic  liquid,  and  the  rapid  suc- 
cession of  generations  soon  gives  rise  to  the  hosts  of  bacteria 
and  Protozoa  which  infest  all  standing  water.  If  all  the 
active  organisms  and  inactive  spores  in  a  glass  of  water  are 
killed  by  boiling  the  water,  "  sterilizing  "  it,  as  it  is  called, 
and  this  sterilized  water  or  organic  liquid  be  put  into  a 
sterilized  glass,  and  this  glass  be  so  well  closed  that  germs 
or  spores  can  not  pass  from  the  air  without  into  the  steril- 
ized liquid,  no  living  animals  will  ever  appear  in  it.  It  is 
now  known  that  flesh  will  not  decay  or  liquids  ferment 
except  through  the  presence  of  living  animals  or  plants. 
To  sum  up,  we  may  say  that  we  know  of  no  instance  of  the 


THE  MULTIPLICATION  OF  ANIMALS  AND  SEX       53 

spontaneous  generation  of  organisms,  and  that  all  the  ani- 
mals whose  life  history  we  know  are  produced  from  other 
animals  of  the  same  kind.  "  Omne  vivum  ex  vivo"  All  life 
from  life. 


FIG.  26.- The  multiplication  of  Amoeba  by  simple  fission. 

30.  The  simplest  method  of  multiplication. — In  our  study 

of  the  simplest  and  the  slightly  complex  animals  we  became 

acquainted  with  the  simplest   methods  of  multiplication 

and  with  methods  which  are  more  complex.     The  method 

5 


54;  ANIMAL   LIFE 

of  simple  fission  or  splitting — binary  fission  it  is  often  called, 
because  the  division  is  always  in  two — by  which  the  body 
of  the  parent  becomes  divided  into  two  equal  parts— into 
halves — is  the  simplest  method  of  multiplication.  This  is 
the  usual  method  of  Amoeba  (Fig.  26)  and  of  many  other  of 
the  simplest  animals.  In  this  kind  of  reproduction  it  is 
hardly  exact  to  speak  of  parent  and  children.  The  chil- 
dren, the  new  Amoebae,  are  simply  the  parent  cut  into 
halves.  The  parent  persists ;  it  does  not  produce  off- 
spring and  die.  Its  whole  body  continues  to  live.  The 
new  AmcBbce  take  in  and  assimilate  food  and  add  new  mat- 
ter to  the  original  matter  of  the  parent  body ;  then  each 
of  them  divides  in  two.  The  grandparent's  body  is  now 
divided  into  four  parts,  one  fourth  of  it  forming  one  half 
of  each  of  the  bodies  of  the  four  grandchildren.  The  pro- 
cess of  assimilation,  growth,  and  subsequent  division  takes 
place  again,  and  again,  and  again.  Each  time  there  is  given 
to  the  new  Amceba  an  ever-lessening  part  of  the  actual 
body  substance  of  the  original  ancestor.  Thus  an  Amoeba 
never  dies  a  natural  death,  or,  as  has  been  said,  "  no  Amoeba 
ever  lost  an  ancestor  by  death."  It  may  be  killed  outright, 
but  in  that  case  it  leaves  no  descendants.  If  it  is  not  killed 
before  it  produces  new  Amoebce  it  never  dies,  although  it 
ceases  to  exist  as  a  single  individual.  The  Amoeba  and 
other  simple  animals  which  multiply  by  direct  binary 
fission  may  be  said  to  be  immortal,  snd  the  "immortality 
of  the  Protozoa  "  is  a  phrase  which  you  will  be  sure  to  meet 
if  you  begin  to  read  the  writings  of  the  modern  philosoph- 
ical zoologists. 

31.  Slightly  complex  methods  of  multiplication.— Most  of 
the  Protozoa  multiply  or  reproduce  themselves  in  two 
ways — by  simple  fission  and  by  conjugation.  Paramce- 
cium,  for  example,  reproduces  itself  for  many  generations 
by  fission,  but  a  generation  finally  appears  in  which  a  dif- 
ferent method  of  reproduction  is  followed.  Two  individu- 
als come  together  and  each  exchanges  with  the  other  a  part 


THE  MULTIPLICATION  OF  ANIMALS  AND  SEX       55 

of  its  nucleus.  Then  the  two  individuals  separate  and 
each  divides  into  two.  The  result  of  this  conjugation  is 
to  give  to  the  new  Paramcecia  produced  by  the  conjugat- 
ing individuals  a  body  which  contains  part  of  the  body 
substance  of  two  distinct  individuals.  The  new  Paramce- 
cia are  not  simply  halves  of  a  single  parent ;  they  are  parts 
of  two  parents.  If  the  two  conjugating  individuals  differ 
at  all — and  they  always  do  differ,  because  no  two  individual 
animals,  although  belonging  to  the  same  species,  are  exactly 
alike — the  new  individual,  made  up  of  parts  of  each  of  them, 
will  differ  from  both.  We  shall,  as  we  study  further,  see 
that  Nature  seems  intent  on  making  every  new  individual 
differ  slightly  from  the  individual  which  produces  it ;  and 
the  method  of  multiplication  or  the  production  of  new  indi- 
viduals which  Nature  has  adopted  to  produce  the  result  is 
the  method  which  we  have  seen  exhibited  in  its  simplest 
form  among  the  simplest  animals — the  method  of  having 
two  individuals  take  part  in  the  production  of  a  new  one. 
The  further  study  of  multiplication  among  animals  is  the 
study  of  the  development  and  elaboration  of  this  method. 
32.  Differentiation  of  the  reproductive  cells. — Among  the 
colonial  Protozoa  the  first  differentiation  of  the  cells  or 
members  composing  the  colony  is  the  differentiation  into 
two  kinds  of  reproductive  cells.  Reproduction  by  simple 
division,  without  preceding  conjugation,  can  and  does  take 
place,  to  a  certain  extent,  among  all  the  colonial  Protozoa. 
Indeed,  this  simple  method  of  multiplication,  or  some  modi- 
.fication  of  it,  like  budding,  persists  among  many  of  the  com- 
plex animals,  as  the  sponges,  the  polyps,  and  even  higher 
and  more  complex  forms.  But  such  a  method  of  single- 
parent  reproduction  can  not  be  used  alone  by  a  species  for 
many  generations,  and  those  animals  which  possess  the 
power  of  multiplication  in  this  way  always  exhibit  also  the 
other  more  complex  kind  of  multiplication,  the  method  of 
double-parent  reproduction.  Conjugation  takes  place  be- 
tween different  members  of  a  single  colony  of  one  of  the 


56  ANIMAL  LIFE 

colonial  Protozoa,  or  between  members  of  different  colonies 
of  the  same  species.  These  conjugating  individuals  in  the 
simpler  kinds  of  colonies,  like  Gonium,  are  similar;  in 
Pandorina  they  appear  to  be  slightly  different,  and  in  Eudo- 
rina  and  Volvox  the  conjugating  cells  are  very  different  from 
each  other  (Figs.  15  and  16).  One  kind  of  cell,  which  is 
called  the  egg  cell,  is  large,  spherical,  and  inactive,  while 
the  other  kind,  the  sperm  cell,  is  small,  with  ovoid  head 
and  tapering  tail,  and  free-swimming.  In  the  simpler  colo- 
nial Protozoa  all  the  cells  of  the  body  take  part  in  repro- 
duction, but  in  Volvox  only  certain  cells  perform  this  func- 
tion, and  the  other  cells  of  the  body  die.  Or  we  may  say 
that  the  body  of  Volvox  dies  after  it  has  produced  special 
reproductive  cells  which  shall  fulfill  the  function  of  multi- 
plication. 

Beginning  with  the  more  complex  Volvocinae,  which  we 
may  call  either  the  most  complex  of  the  one-celled  animals 
or  the  simplest  of  the  many-celled  animals,  all  the  cjpplex 
animals  show  this  distinct  differentiation  between  ^ie  re- 
productive cells  and  the  cells  of  the  rest  of  the  body.  Of 
course,  we  find,  as  soon  as  we  go  up  at  all  far  in  the  scale  of 
the  animal  world,  that  there  is  a  great  deal  of  differentia- 
tion among  the  cells  of  the  body :  the  cells  which  have  to 
do  with  the  assimilation  of  food  are  of  one  kind ;  those  on 
which  depend  the  motions  of  the  body  are  of  another  kind ; 
those  which  take  oxygen  and  those  which  excrete  waste 
matter  are  of  other  kinds.  But  the  first  of  this  cell  differ- 
entiation, as  we  have  already  often  repeated,  is  that  shown 
by  the  reproductive  cells ;  and  with  the  very  first  of  this 
differentiation  between  reproductive  cells  and  the  other 
body  cells  appears  a  differentiation  of  the  reproductive 
cells  into  two  kinds;  These  two  kinds,  among  all  animals, 
are  always  essentially  similar  to  the  two  kinds  shown  by 
Volvox  and  the  simplest  of  the  many-celled  animals — namely, 
large,  inactive,  spherical  egg  cells,  and  small,  active,  elon- 
gate or  "  tailed  "  sperm  cells. 


THE  MULTIPLICATION  OF  ANIMALS  AND  SEX       57 

33.  Sex,  or  male  and  female. — In  the  slightly  complex 
animals  one  individual  produces  both  egg  cells  and  sperm 
cells.    But  in  the  Siphonophora,  or  colonial  jelly-fishes,  stud- 
ied in  the  last  chapter,  certain  members  of  the  colony  pro- 
duce only  sperm  cells,  and  certain  other  members  of  the 
colony  produce  only  egg  cells.     If  the  Siphonophora  be 
considered  an  individual  organism  and  not  a  colony  com- 
posed of  many  individuals,  then,  of  course,  it  is  like  the 
others  of  the  slightly  complex  animals  in  this  respect.     But 
as  soon  as  we  rise  higher  in  the  scale  of  animal  life,  as  soon 
as  we  study  the  more  complex  animals,  we  find  that  the 
egg  cells  and  sperm  cells  are  almost  always  produced  by 
different  individuals.      Those    individuals  which  produce 
egg  cells  are  called  female,  and  those  which  produce  sperm 
cells  are  called  male.     There  are  two  sexes.     Male  and 
female  are  terms  usually  applied  only  to  individuals,  but 
it  is  evidently  fair  to  call  the  egg  cells  the  female  reproduc- 
tive cells,  and  the  sperm  cells  the  male  reproductive  cells. 
A  single  individual  of  the  simpler  kinds  of  animals  pro- 
duces both  male  and  female  cells.     But  such  an  individual 
can  not  be  said  to  be  either  male  or  female ;  it  is  sexless — 
that  is,  sex  is  something  which  appears  only  after  a  certain 
degree  of   structural   and  physiological  differentiation  is 
reached.     It  is  true  that  even  among  many  of  the  higher 
or  complex  animals  certain  species  are  not  represented  by 
male  and  female  individuals,  any  individual  of  the  species 
being  able  to  produce  both  male  and  female  cells.    But  this 
is  the  exception. 

34.  The  object  of  sex.— Among  almost  all  the  complex 
animals  it  is  necessary  that  there  be  a  conjugation  of  male 
and  female  reproductive  cells  in  order  that  a  new  individual 
may  be  produced.     This  necessity  first  appears,  we  remem- 
ber, among  very  simple  animals.     This  intermixing  of  body 
substance  from  two  distinct  individuals,  and  the  develop- 
ment therefrom  of  the  new  individual,  is  a  phenomenon 
which  takes  place  through  the  whole  scale  of  animal  life. 


58 


ANIMAL  LIFE 


The  object  of  this  intermixing  is  the  production  of  va- 
riation. Nature  demands  that  the  offspring  shall  differ 
slightly  from  its  parents.  By  having  the  beginnings  of  its 
body,  the  single  cell  from  which  the  whole  body  develops, 
composed  of  parts  of  two  different  individuals,  this  differ- 
ence, although  slight  and  nearly  imperceptible,  is  insured. 
Sex  is  a  provision  of  Nature  which  insures  variation. 

35.  Sex  dimorphism. — As  we  have  seen,  almost  every 
species  of  animal  is  represented  by  two  kinds  of  individuals, 
males  and  females.  In  the  case  of  many  animals,  espe- 


Fm.  27.— Bird  of  paradise,  male. 


cially  the  simpler  ones,  these  two  kinds  of  individuals  do 
not  differ  in  appearance  or  in  structure  apart  from  the 
organs  concerned  with  multiplication.  But  with  many 
animals  the  sexes  can  be  readily  distinguished.  The  male 
and  female  individuals  often  show  marked  differences, 
especially  in  external  structural  characters.  We  can  read- 


THE  MULTIPLICATION  OF  ANIMALS  AND  SEX       59 

ily  tell  the  peacock,  with  its  splendidly  ornamental  tail 
feathers,  from  the  unadorned  peafowl,  or  the  horned  ram 
from  the  bleating  ewe.  There  is  here,  plainly,  a  dimor- 
phism— the  existence  of  two  kinds  of  individuals  belonging 
to  a  single  species.  This  dimorphism  is  due  to  sex,  and 
the  condition  may  be  called  sex  dimorphism.  Among  some 
animals  this  sex  dimorphism,  or  difference  between  the 
sexes,  is  carried  to  extraordinary  extremes.  This  is  espe- 
cially true  among  polygamous  animals,  or  those  in  which 
the  males  mate  with  many  females,  and  are  forced  to  fight 
for  their  possession.  The  male  bird  of  paradise,  with  its 
gorgeous  display  of  brilliantly  colored  and  fantastically 
shaped  feathers  (Fig.  27),  seems  a  wholly  different  kind  of 
bird  from  the  modest  brown  female.  The  male  golden  and 
silver  pheasants,  and  allied  species  with  their  elaborate 
plumage,  are  very  unlike  the  dull-colored  females.  The 
great,  rough,  warlike  male  fur  seal,  roaring  like  a  lion,  is 
three  times  as  large  as  the  dainty,  soft-furred  female,  which 
bleats  like  a  sheep. 

Among  some  of  the  lower  animals  the  differences  be- 
tween male  and  female  are  even  greater.  The  males  of 
the  common  cankerworm  moth  (Fig.  28)  have  four  wings ; 


FIG.  £8. — Cankerworm  moth ;  the  winged  male  and  wingless  female. 

the  females  are  wingless,  and  several  other  insect  species 
show  this  same  difference.  Among  certain  species  of  white 
ants  the  females  grow  to  be  five  or  six  inches  long,  while 
the  males  do  not  exceed  half  an  inch  in  length.  In  the 


60 


ANIMAL  LIFE 


case  of  some  of  the  parasitic  worms  which  live  in  the  bod- 
ies of  other  animals,  the  male  has  an  extraordinarily  de- 
graded, simple  body,  much  smaller  than  that  of  the  female 
and  differing  greatly  from  that  of  the  female  in  structure. 
In  some  cases  even — as,  for  example, 
the  worm  which  causes  "  gapes  "  in 
chickens  —  the  male  lives  parasiti- 
cally  on  the  female,  being  attached  to 
the  body  of  the  female  for  its  whole 
lifetime,  and  drawing  its  nourish- 
ment from  her  blood  (Fig.  29). 

A  condition  known  as  partheno- 
genesis is  found  among  certain  of 
the  complex  animals.  Although  the 
species  is  represented  by  individu- 
als of  both  sexes,  the  female  can 
produce  young  from  eggs  which 
have  not  been  fertilized.  For  ex- 
ample, the  queen  bee  lays  both  fer- 
tilized and  unfertilized  eggs.  From 
the  fertilized  eggs  hatch  the  work- 
ers, which  are  rudimentary  females, 
and  other  queens,  which  are  fully- 
29.-The  parasitic  worm  developed  females ;  from  the  unfer- 

(Syngamus  trachealis),  which  r 

causes  the  "gapes  "in  fowls,  tilized  eggs  hatch  only  males— the 
The  male  is  attached  to  the  drones.     Many  generations  of  plant 

female,  and  lives  as  a  para-    ,. 

site  on  her.  lice  are  produced  each  year  parthe- 

nogenetically  —  that  is,  by  unferti- 
lized females.  But  there  is  at  least  one  generation  each 
year  produced  in  the  normal  way  from  fertilized  eggs. 

Some  of  the  complex  animals  are  hermaphroditic — that 
is,  a  single  individual  produces  both  egg  cells  and  sperm 
cells.  The  tapeworm  and  many  allied  worms  show  this 
condition.  This  is  the  normal  condition  for  the  simplest 
animals,  as  we  have  already  learned,  but  it  is  an  excep- 
tional condition  among  the  complex  animals. 


THE  MULTIPLICATION  OF  ANIMALS  AND  SEX       61 

36.  The  number  of  young. — There  is  great  variation  in  the 
number  of  young  produced  by  different  species  of  animals. 
Among  the  animals  we  know  familiarly,  as  the  mammals, 
which  give  birth  to  young  alive,  and  the  birds,  which  lay 
eggs,  it  is  the  general  rule  that  but  few  young  are  pro- 
duced at  a  time,  and  the  young  are  born  or  eggs  are.  laid 
only  once  or  perhaps  a  few  times  in  a  year.  The  robin  lays 
five  or  six  eggs  once  or  twice  a  year ;  a  cow  may  produce 
a  calf  each  year.  Babbits  and  pigeons  are  more  prolific, 
each  having  several  broods  a  year.  But  when  we  observe 
the  multiplication  .of  some  of  the  animals  whose  habits  are 
not  so  familiar  to  us,  we  find  that  the  production  of  so  few 
young  is  the  exceptional  and  not  the  usual  habit.  A  lob- 
ster lays  ten  thousand  eggs  at  a  time ;  a  queen  bee  lays 
about  five  million  eggs  in  her  life  of  four  or  five  years.  A 
female  white  ant,  which  after  it  is  full  grown  does  nothing 
but  lie  in  a  cell  and  lay  eggs,  produces  eighty  thousand 
eggs  a  day  steadily  for  several  months.  A  large  codfish 
was  found  on  dissection  to  contain  about  eight  million' 


If  we  search  for  some  reason  for  this  great  difference  in 
fertility  among  different  animals,  we  may  find  a  promis- 
ing clew  by  attending  to  the  duration  of  life  of  animals, 
and  to  the  amount  of  care  for  the  young  exercised  by  the 
parents.  We  find  it  to  be  the  general  rule  that  animals 
which  live  many  years,  and  which  take  care  of  their  young, 
produce  but  few  young ;  while  animals  which  live  but  a 
short  time,  and  which  do  not  care  for  their  young,  are  very 
prolific.  The  codfish  produces  its  millions  of  eggs ;  thou- 
sands are  eaten  by  sculpins  and  other  predatory  fishes  be- 
fore they  are  hatched,  and  other  thousands  of  the  defense- 
less young  fish  are  eaten  long  before  attaining  maturity. 
Of  the  great  number  produced  by  the  parent,  a  few  only 
reach  maturity  and  produce  new  young.  But  the  eggs  of  the 
robin  are  hatched  and  protected,  and  the  helpless  fledglings 
are  fed  and  cared  for  until  able  to  cope  with  their  natural 


62  ANIMAL  LIFE 

enemies.     In  the  next  year  another  brood  is  carefully  reared, 
and  so  on  for  the  few  years  of  the  robin's  life. 

Under  normal  conditions  in  any  given  locality  the  num- 
ber of  individuals  of  a  certain  species  of  animal  remains 
about  the  same.  The  fish  which  produces  tens  of  thousands 
of  eggs  and  the  bird  which  reproduces  half  a  dozen  eggs  a 
year  maintain  equally  well  their  numbers.  In  one  case  a 
few  survive  of  many  born ;  in  the  other  many  (relatively) 
survive  of  the  few  born  ;  in  both  cases  the  species  is  effect- 
ively maintained.  In  general,  no  agency  for  the  perpetua- 
tion of  the  species  is  so  effective  as  that  of  care  for  the 
young. 


CHAPTER  IV 

FUNCTION  AND   STRUCTURE 

37.  Organs  and  functions. — An  animal  does  certain  things 
which  are  necessary  to  life.  It  eats  and  digests  food,  it 
breathes  in  air  and  takes  oxygen  from  it  and  breathes  out 
carbonic-acid  gas ;  it  feels  and  has  other  sensations ;  it  pro- 
duces offspring,  thus  reproducing  itself.  These  things  are 
done  by  the  simplest  animals  as  well  as  by  the  complex 
animals.  But  while  with  the  simplest  animals  the  whole 
body  (which  is  but  a  single  cell)  takes  part  in  doing  each 
of  these  things,  among  the  complex  animals  only  a  part 
of  the  body  is  concerned  with  any  one  of  these  things. 
Only  a  part  of  the  body  has  to  do  with  the  taking  in  of 
oxygen.  Another  part  has  to  do  with  the  digestion  of 
food,  and  another  with  the  business  of  locomotion.  These 
parts  of  the  body,  as  we  know,  differ  from  each  other,  and 
they  differ  because  they  have  different  things  to  do.  These 
different  parts  are  called  organs  of  the  body,  and  the  things 
they  do  are  called  their  functions.  The  nostrils,  tracheae, 
and  lungs  are  the  organs  which  have  for  function  the  pro- 
cess of  respiration.  The  legs  of  a  cat  are  the  organs  which 
perform  for  it  the  function  of  locomotion.  The  structure 
of  one  of  the  higher  animals  is  complex  because  the  body 
is  made  up  of  many  distinct  organs  having  distinct  func- 
tions. The  things  done  by  one  of  the  complex  animals  are 
many ;  around  each  of  the  principal  functions  or  necessary 
processes,  as  a  center,  are  grouped  many  minor  accessory 
functions,  all  helping  to  make  more  successful  the  accom- 

63 


64.  ANIMAL  LIFE 

plishment  of  the  principal  functions.  While  many  of  the 
lower  animals  have  no  eyes  and  no  ears,  and  trust  to  more 
primitive  means  to  discover  food  or  avoid  enemies,  the 
higher  animals  have  extraordinarily  complex  organs  for 
seeing  and  hearing,  two  functions  which  are  accessory  only 
to  such  a  principal  function  as  food-taking. 

38.  Differentiation  of  structure, — We  have  seen,  in  our 
study  of  the  slightly  complex  animals,  how  the  body  be- 
comes more  and  more  complex  in  proportion  to  the  degree 
in  which  the  different  life  processes  are  divided  or  assigned 
to  different  parts  of  it  for  performance.     With  the  gradu- 
ally increasing  division  of  labor  the  body  becomes  less 
homogeneous  in  structure;  a  differentiation  of  structure 
becomes  apparent  and  gradually  increases.     The  extent  of 
the  division  of  labor  and  the  extent  of  the  differentiation 
of  structure,  or  division  of  the  body  into  distinct  and  dif- 
ferent parts  and  organs,  go  hand  in  hand.     An  animal  in 
which  the  division  of  labor  is  carried  to  an  extreme  is  an 
animal  in  which  complexity  of  structure  is  extreme. 

39.  Anatomy  and  physiology. — Zoology,  or  the  study  of 
animals,  is  divided  for  convenience  into  several  branches 
or  phases.     The  study  of  the  classification  of  animals  is 
called  systematic  zoology;  the  study  of  the  development 
of  animals  from  their  beginning  as  a  single  cell  to  the  time 
of  their  birth  is  called  animal  embryology ;  the  study  of 
the  structure  of  animals  is  called  animal  anatomy,  and  the 
study  of  the  performance  of  their  life  processes  or  functions 
is  called  physiology.     Because  the  whole  field  of  zoology  is 
so  great,  some  zoologists  limit  themselves  exclusively  to  one 
of  these  phases  of  zoological  study,  and  those  who  do  not 
so  definitely  limit  their  study,  at  least  give  their  special  at- 
tention to  a  single  phase,  although  all  try  to  keep  in  touch 
with  the  state  of  knowledge  in  other  phases.     In  earlier 
days  the  study  of  the  anatomy  of  animals  and  of  their 
physiology  were  held  to  be  two  very  distinct  lines  of  in- 
vestigation, and  the  anatomists  paid  little  attention  to 


FUNCTION  AND  STRUCTURE  65 

physiology  and  the  physiologists  little  to  anatomy.  But 
we  have  seen  how  inseparably  linked  are  structure  and 
function.  The  structure  of  an  animal  is  as  it  is  because 
of  the  work  it  has  to  do,  and  the  functions  of  an  animal 
are  performed  as  they  are  performed  because  of  the  special 
structural  condition  of  the  organs  which  perform  them. 
The  study  of  the  anatomy  and  the  study  of  the  physiology 
of  animals  can  not  be  separated.  To  understand  aright 
the  structure  of  an  animal  it  is  necessary  to  know  to 
what  use  the  structure  is  put ;  to  understand  aright  the 
processes  of  an  animal  it  is  necessary  to  know  the  struc- 
ture on  which  the  performance  of  the  processes  depends. 

40.  The  animal  body  a  machine. — The  body  of  an  animal 
may  be  well  compared  with  some  machine  like  a  locomotive 
engine.  Indeed,  the  animal  body  is  a  machine.  It  is  a 
machine  composed  of  many  parts,  each  part  doing  some 
particular  kind  of  work  for  which  a  particular  kind  of 
structure  fits  it ;  and  all  the  parts  are  dependent  on  each 
other  and  work  together  for  the  accomplishment  of  the 
total  business  of  the  machine.  The  locomotive  must  be 
provided  with  fuel,  such  as  coal  or  wood  or  other  readily 
combustible  substance,  the  consumption  of  which  furnishes 
the  force  or  energy  of  the  machine.  The  animal  body 
must  be  provided  with  fuel,  which  is  called  food,  which 
furnishes  similarly  the  energy  of  the  animal.  Oxygen  must 
be  provided  for  the  combustion  of  the  fuel  in  the  locomo- 
tive and  the  food  in  the  body.  The  locomotive  is  com- 
posed of  special  parts :  the  firebox  for  the  reception  and 
combustion  of  fuel ;  the  steam  pipes  for  the  carriage  of 
steam  ;  the  wheels  for  locomotion ;  the  smoke  stack  for 
throwing  off  of  waste.  The  animal  body  is  similarly  com- 
posed of  parts  :  the  alimentary  canal  for  the  reception  and 
assimilation  of  food  ;  the  excretory  organs  for  the  throwing 
off  of  waste  matter  ;  the  arteries  and  veins  for  the  carriage 
of  the  oxygen  and  food-holding  blood ;  the  legs  or  wings 
for  locomotion. 


66  ANIMAL  LIFE 

The  locomotive  is  an  inorganic  machine  ;  the  animal  is 
an  organic  machine.  There  is  a  great  and  real  difference 
between  an  organism,  a  living  animal,  and  a  locomotive,  an 
inorganic  structure.  But  for  a  good  understanding  of  the 
relation  between  function  and  structure,  and  of  the  com- 
position of  the  body  of  the  complex  animals,  the  compari- 
son of  the  animal  and  locomotive  is  very  instructive. 

41.  The  specialization  of  organs.— The  organ  for  the  per- 
formance of  some  definite  function  in  one  of  the  higher 
animals  may  be  very  complex.     The  corresponding  organ 
in  one  of  the  lower  animals  for  the  performance  of  the 
same  function  may  be  comparatively  simple.     For  example, 
the  organ  for  the  digestion  of  food  is,  in  the  case  of  the 
polyp,  a  simple  cylindrical  cavity  in  the  body  into  which 
food  enters  through  a  large  opening  at  the  apical  or  free 
end  of  the  body.     The  digestive  organ  of  a  cow  is  a  long 
coiled  tube,  comprising  many  regions  of  distinct  structural 
and  physiological  character  and  altogether  extremely  com- 
plicated.    An  organ  in  simple  or  primitive  condition  is 
said  to  be  generalized;  in  complex  or  highly  modified  con- 
dition it  is  said  to  be  specialized.     That  is,  an  organ  may 
be  modified  and  complexly  developed  to  perform  its  func- 
tion in  a  special  way,  in  a  way  differing  in  many  particu- 
lars from  the  way  the  corresponding  organ  in  some  other 
animal  performs  the  same  general  function.     The  speciali- 
zation of  organs,  or  their  modification  to  perform  their 
functions  in  special  ways,  is  what  makes  animal  bodies 
complex,  for  specialization  is  almost  always  in  the  line  of 
complexity.     Later  we  shall  see  more  clearly  how  specializa- 
tion is  brought  about.      For  the  present  we   may  study 
one  of  the  more  important  organs  of  the  animal  body  for 
the  sake  of  having  concrete  examples  of  some  of  the  gen- 
eral statements  made  in  this  discussion  of  function  and 
structure. 

42.  The  alimentary  canal — The  organ  which  has  to  do 
with  the  taking  and  digesting  of  food  is  called  the  ali- 


FUNCTION  AND  STRUCTURE 


mentary  canal.  In  some  of  the  higher  animals  this  is  a 
very  complex  organ.  In  the  cow,  one  of  the  cud-chewing 
mammals  or  ruminants,  it  consists  of  several  distinct  por- 
tions, which  differ  among  themselves  very  much  (Fig.  30). 
First,  there  is  the  mouth,  or  opening  for  the  entrance  of 
the  food.  The  mouth  is  sup- 
plied with  teeth  for  tearing 
off  and  chewing  the  food, 
with  a  tongue  for  manipu- 
lating it,  and  with  taste  pa- 
pillae situated  on  the  tongue 
and  palate  for  determining 
the  desirability  of  the  food. 
Into  the  mouth  a  peculiar 
fluid  (the  saliva)  is  poured 
by  certain  glands,  organs  ac- 
cessory to  the  alimentary 
canal.  The  herbage  bitten 
off,  mixed  with  saliva,  and 
rolled  by  the  tongue  into  a 
ball,  passes  back  through  a 
narrow  tube,  the  oesophagus, 
and  into  a  sac  called  the  ru- 
men, or  paunch.  Here  it 
lies  until  the  cow  ceases  for 
the  while  to  take  in  food, 
when  it  passes  back  again 
through  the  oesophagus  and 
into  the  mouth  for  mastica- 
tion. After  being  masticated  it  again  passes  downward 
through  the  oesophagus,  and  enters  this  time  another  sac 
called  the  reticulum,  lying  next  to  the  rumen.  From  here 
it  passes  into  another  sac-like  portion  of  the  alimentary 
canal  called  the  omasum,  where  it  is  strained  throng1! 
numerous  leaf-like  folds  which  line  the  walls  of  this  part 
of  the  canal.  From  here  the  food  passes  into  a  fourth 


FIG.  30.— Alimentary  canal  of  the  ox 
(after  COLIN  and  MULLER).  a,  rumen 
(left  hemsiphere)  ;  b,  rumen  (right  hem- 
isphere) ;  c,  insertion  of  oesophagus  ;  d, 
reticulum  ;  0,  omasum  ;  f,  abomasum  ; 
g,  duodenum ;  h  and  i,  jejunum  and 
ileum ;  j,  caecum ;  k,  colon,  with  its 
various  convolutions  ;  I,  rectum. 


68  ANIMAL  LIFE 

sac-like  part  of  the  canal,  called  the  abomasum.  Here 
the  process  of  digestion  goes  on.  The  four  cacs — rumen, 
reticulum,  omasum,  and  abomasum — are  called  stomachs, 
or  they  may  be  considered  to  be  four  chambers  forming 
one  large  stomach.  In  the  abomasum,  or  digesting  stom- 
ach, digestive  fluids  are  poured  from  glands  lining  its 
walls,  and  the  food  becomes  converted  into  a  liquid  called 
chyle.  The  chyle  passes  from  the  stomach  into  a  long, 
narrow,  tubular  portion  of  the  canal  called  the  intestine. 
The  intestine  is  very  long,  and  lies  coiled  in  a  large  mass 
in  the  body  of  the  cow.  The  intestine  is  divided  into 
distinct  regions,  which- vary  in  size  and  in  the  character 
of  the  inner  wall.  These  parts  of  the  intestine  have 
names,  as  duodenum,  jejunum,  ileum,  caecum,  colon,  etc. 
Part  of  the  intestine  is  lined  inside  with  fine  papillae, 
which  take  up  the  chyle  (the  digested  food)  and  pass  it 
through  the  walls  of  the  intestine  to  other  special  organs, 
which  pass  it  on  to  the  blood,  with  which  it  becomes  mixed 
and  carried  by  an  elaborate  system  of  tubes  to  all  parts  of 
the  body.  Part  of  the  grass  taken  into  the  alimentary 
canal  by  the  cow  can  not  be  digested,  and  must  be  got  rid 
of.  This  passes  on  into  a  final  posterior  part  of  the  intes- 
tine called  the  rectum,  and  leaves  the  body  through  the 
anus  or  posterior  opening  of  the  alimentary  canal.  The 
whole  canal  is  more  than  twenty  times  as  long  as  the  body 
of  the  cow ;  it  is  composed  of  parts  of  different  shape  ;  its 
walls  are  supplied  with  muscles  and  blood-vessels  ;  the  inner 
lining  is  covered  with  folds,  papillae,  and  gland  cells.  It  is 
altogether  a  highly  specialized  organ,  a  structurally  com- 
plex and  elaborately  functioning  organ. 

Let  us  now  examine  the  alimentary  canal,  or  organ  of 
digestion,  in  some  of  the  simpler  animals. 

The  Protozoa,  or  simplest  animals,  have  no  special  organ 
at  all.  When  the  surface  of  the  body  of  an  Amceba  comes 
into  contact  with  an  organic  particle  which  will  serve  as 
food,  the  surface  becomes  bent  in  at  the  point  of  its  con- 


FUNCTION  AND  STEUCTURE 


69 


tact  with  the  food  particle,  and  the  body  substance  simply 
incloses  the  food  (Fig.  3).  Food  is  taken  in  by  the  sur- 
face. The  whole  outer  surface  of  the  body  is  the  food- 
taking  organ.  In  the  simplest  many-celled  animals,  the 
sponges,  there  is  no  special  food-taking  and  digestive  organ. 
Each  of  the  cells  of  the  body  takes  in  and  assimilates  food 
for  itself.  The  sponge  is  like  a  great  group  of  Amcebce 
holding  fast  to  each  other,  but  each  looking  out  for  its  own 
necessities.  Among  the  m 

polyps,  however,  there 
is  a  definite  organ  of 
digestion — that  is,  food 
is  only  taken  and  di- 
gested by  certain  parts 
of  the  body.  The  sim- 
ple polyp's  body  (Fig. 
31)  is  a  cylinder  or  vase 
closed  at  one  end  and 
open  at  the  other  end, 
and  attached  by  the 
closed  end  to  a  rock. 
The  opening  is  usually 
of  less  diameter  than 


at  s,~ 


the  diameter 
body,  and  it 
rounded  by  a 
of  tentacles, 


of 


FIG.  31. — Obeliasp.iS.  simple  polyp;  vertical  sec- 
tion, highly  magnified,  rn,  mouth  opening ; 
al.  *.,  alimentary  sac.  — After  PARKER  and 
HASWELL. 


the 

is    sur- 
number 
whose 

function  it  is  to  seize  the  food  and  convey  it  to  the  mouth 
opening.  There  are,  of  course,  no  teeth,  no  tongue,  none 
of  the  various  parts  which  are  in  or  are  part  of  the  mouth 
of  the  higher  animals.  The  polyp's  mouth  is  simply  a 
hole  or  opening  into  the  inside  of  the  body.  This  body 
cavity,  or  simplest  of  all  stomachs,  is  simply  the  cylindrical 
or  vase-shaped  hollow  space  inclosed  by  the  body  wall. 
This  space  extends  also  into  the  tentacles.  There  is  no 
other  opening,  no  posterior  or  anal  opening.  We  can  not 
6 


70 


ANIMAL  LIFE 


speak  of  an  oesophagus  or  intestine  in  connection  with  this 
most  primitive  of  alimentary  sacs.  The  cells  which  line 
the  sacs  show  some  differentiation ;  some  are  gland  cells 
and  secrete  digestive  fluids;  some  are  amoeboid  and  are 
provided  with  pseudopods  or  flagella  for  seizing  bits  of 
food.  The  food  caught  by  the  tentacles  comes  into  the  ali- 
mentary sac  through  the  opening  or  primitive  mouth,  and 


.at C 


.  32.— Diagrammatic  sketch  of  a  flat- 
worm  (Planaria),  showing  the 
branched  alimentary  canal,  al.  c.— 
After  JIJIMA  and  HATSCHEK. 


FIG.  as.— Sea-cucumber  (Holothurian) 
dissected  to  show  alimentary  canal, 
al.  c.— After  LEUCKART. 


what  of  it  is  digestible  is,  by  the  aid  of  the  gland  cells  and 
the  amoeboid  cells,  taken  up  and  assimilated,  while  the  rest 
of  it  is  carried  out  by  water  currents  again  through  the 
single  opening. 

In  the  flatworms  (Fig.  32)  like  Planaria  (small,  thin, 
flattened  worms  to  be  found  in  the  mud  at  the  bottom  of 
fresh-water  ponds)  the  mouth  opens  into  a  short,  narrow 
tube  which  may  be  called  an  oesophagus.  The  oesophagus 


FUNCTION  AND  STRUCTURE 


71 


connects  the  mouth  with  the  rest  of  the  alimentary  canal, 
which  gives  out  many  side  branches  or  diverticula,  which 
are  themselves  branched,  so  that  the 
alimentary  sac  or  stomach  is  a  system 
of  ramifying  tubes  extending  from  a 
central  main  tube  to  all  parts  of  the 
body  of  the  worm.  There  is  no 
anal  opening.  In  the  round  or  thread 
worms,  of  which  the  deadly  Trichina 
is  an  example,  the  alimentary  canal 
is  a  simple  straight  tube  with  both 
anterior  or  mouth  opening  and  pos- 
terior or  anal  opening.  In  the  sea- 
urchins  and  sea-cucumbers  (Fig.  33) 
the  alimentary  canal  is  a  simple  tube 
with  two  openings,  but  it  is  longer 
than  the  body  between  mouth  and 
anus,  and  so  is  more  or  less  bent  or 
coiled.  In  the  earthworm  the  ali- 
mentary canal  (Fig.  34),  although  a 
simple  straight  tube  running  through 
the  body,  plainly  shows  a  differentia- 
tion into  particular  regions.  Behind 
the  mouth  opening  the  alimentary 
tube  is  large  and  thick  -  walled  and 
is  called  the  pharynx;  behind  the 
pharynx  it  is  narrower  and  is  called 
the  oesophagus.  Behind  the  oesopha- 
gus it  expands  to  form  a  rounded, 
thin-walled  chamber  called  the  crop, 
and  just  behind  this  there  is  another 

rounded  but  Verv  thick-walled  Cham-  FH»-  34.-Earthworm  dissected 
J   .  T          -n  ji  to  show  alimentary  canal, 

ber   called  the   gizzard.      From  the      ^  c> 

gizzard  back  the  alimentary  canal  is 

about  uniform  in  size,  being  rather  wide  and  having  thick, 

soft  walls.     This  portion  of  it  is  called  the  intestine.     The 


72  ANIMAL  LIFE 

posterior  part  of  the  intestine,  called  the  rectum,  leads  to 
the  anal  opening.  There  is  some  differentiation  of  the 
inner  surface  of  the  canal.  In  the  great  group  of  mol- 
lusks,  of  which  the  common  fresh-water  clam  or  mussel  is 
an  example,  the  alimentary  canal  (Fig.  35)  shows  much 
variation.  The  microscopic  plants,  which  are  the  food  of 
the  mussel,  are  taken  in  through  the  mouth  and  pass  into 
a  short  oesophagus,  thence  into  a  wide  stomach  and  there 
digested.  Behind  the  stomach  is  a  long,  much-folded,  nar- 
row intestine  which  winds  about  through  the  fleshy  "  foot " 
and  finally  reaches  the  surface  of  the  body,  and  has  an 
anal  opening  at  a  point  opposite  the  position  of  the  mouth. 
Among  the  insects  there  is  a  great  range  in  degree  of 
complexity  of  the  alimentary  canal.  The  digestive  organs 
are,  however,  in  most  insects  in  a  condition  of  high  speciali- 
zation. The  mouth  opening  is  provided  with  well-developed 


ctl  C. 

FIG.  35.— Pond  mussel  dissected  to  show  alimentary  canal,  al.  c.— After  HATSCHEK 

and  Com. 

biting  and  masticating  or  piercing  and  sucking  mouth  parts ; 
pharynx,  oesophagus,  stomach,  and  intestine  are  always  dif- 
ferentiated and  sometimes  greatly  modified.  In  the  com- 
mon cockroach,  for  example  (Fig.  36),  the  mouth  has  a 
complicated  food-getting  apparatus,  and  the  canal,  which 


FUNCTION  AND  STRUCTURE 


73 


is  much  longer  than  the  body  of  the  insect,  and  hence 

much  bent  and  coiled,  consists  of  a  pharynx,  oesophagus, 

fore-stomach  or  proventriculus, 

true  digesting  stomach  or  ven- 

triculus,  intestine,  and  rectum 

which   opens   at   the  posterior 

tip  of  the  body.      The   inner 

lining  of  the  canal  shows  much 

differentiation  in  the  different 

parts  of  the  canal,  and  there 

are  numerous  accessory  glands 

connected  with  various  parts  of 

the  canal. 

Finally,  among  the  highest 
animals,  the  vertebrates,  we 
find  still  more  elaborate  special- 
ization of  the  alimentary  canal. 
As  an  example  the  alimentary 
canal  of  a  cow  has  already  been 
described  in  detail. 

43.  Stable  and  variable  char- 
acteristics   of    an    organ.  —  In 
spite  of  all  this  variation  in 

the  structure  and  general  character  of  the  alimentary 
canal,  there  are  certain  characteristics  which  are  features 
of  all  alimentary  canals.  In  the  examination  of  an  organ 
we  must  ever  distinguish  between  its  so-called  constant  or 
stable  characteristics  and  its  inconstant  or  variable  charac- 
teristics. The  constant  characteristics  are  the  fundamen- 
tally essential  ones  of  the  organ ;  the  variable  ones  are  the 
special  characteristics  which  adapt  the  organ  for  the  pecul- 
iar habits  of  the  animal  possessing  it — habits  which  may 
differ  very  much  from  those  of  some  other  animal  of  similar 
size,  similar  distribution,  similar  abundance. 

44.  Stable  and  variable  characteristics  of  the  alimentary 
canal. — A  tiger  or  a  lion  has  an  alimentary  canal  not  more 


die 


FIG.  36.— Cockroach  dissected  to  show 
alimentary  canal,  al.  c.— After  HAT- 
SCHEK  and  Com. 


74  ANIMAL  LIFE 

than  three  or  four  times  the  length  of  its  body,  while  a 
sheep  has  an  alimentary  canal  twenty-eight  times  as  long 
as  its  body.  The  tiger  is  carnivorous;  the  sheep  her- 
bivorous. Associated  with  the  different  food  habits  of  the 
two  animals  is  a  striking  difference  in  the  alimentary 
canals.  Animals  like  the  horse  or  cat,  which  chew  their 
food  before  swallowing  it,  have  a  slender  oesophagus ;  ani- 
mals like  snakes  which  swallow  their  food  whole  have  a 
wide  oesophagus.  Birds,  that  have  no  teeth  and  hence 
can  not  masticate  or  grind  their  food  in  their  mouths,  usu- 
ally have  a  special  grinding  stomach,  the  gizzard,  for  this 
purpose.  And  so  we  might  cite  innumerable  examples 
of  these  inconstant  or  variable  characteristics  of  the  ali- 
mentary canal.  On  the  other  hand,  the  alimentary  canals 
of  all  the  many-celled  animals  except  the  lowest  agree  in 
certain  important  characteristics.  Each  alimentary  canal 
has  two  openings,  one  for  the  ingress  of  food  and  one  for 
the  exit  of  the  indigestible  portions  of  the  matter  taken  in, 
and  the  canal  itself  stretches  through  the  body  from  mouth 
to  anus  as  a  tube,  now  narrow,  now  wide,  now  suddenly 
expanding  into  a  sac  or  giving  off  lateral  diverticula,  but 
always  simply  a  lumen  or  hollow  inclosed  by  a  flexible  mus- 
cular wall.  The  inner  lining  of  the  wall  is  provided  with 
secreting  and  absorbing  structures.  Indeed,  we  can  reduce 
the  essential  characters  of  the  alimentary  canal  to  even 
more  simple  features.  The  organ  of  digestion  or  assimila- 
tion of  all  the  many-celled  animals  is  merely  a  surface  with 
which  food  is  brought  into  contact,  and  which  has  the 
power  of  digesting  this  food  by  means  of  digestive  secre- 
tions, and  of  absorbing  the  food  when  digested.  This  sur- 
face is  small  or  great  in  extent,  depending  upon  the  amount 
of  food  necessary  to  the  life  of  the  animal  and  the  difficulty 
or  readiness  with  which  the  food  can  be  digested.  This 
surface  might  just  as  well  be  on  the  outside  of  the  animal's 
body  as  on  the  inside,  if  it  were  convenient.  In  fact,  it  is 
on  the  outside  of  some  animals.  Among  the  Protozoa  the 


FUNCTION  AND  STRUCTURE  Y5 

digesting  surface  is  simply  the  external  surface  of  the  body. 
And  not  alone  among  the  one-celled  animals.  Many  of  the 
parasitic  worms  which  live  in  the  bodies  of  other  animals, 
and  the  larvae  or  "  grubs  "  of  many  insects  which  lie  in  the 
tissues  of  plants  bathed  by  the  sap,  have  no  inner  alimen- 
tary canal,  but  take  food  through  the  outer  surface  of  the 
body.  But  in  these  cases  the  food  is  ready  for  immediate 
absorption,  so  that  no  special  treatment  of  it  is  necessary, 
hence  no  complex  structures  are  required. 

Even  were  no  such  special  treatment  of  the  food  neces- 
sary in  the  case  of  the  larger  animals,  it  would  still  be  im- 


Fio.  37.— Diagram  illustrating  increase  of  volume  and  surface  with  increase  of 
diameter  of  sphere. 

possible  for  the  simple  external  surface  of  the  body  to  serve 
for  food  absorption,  because  of  the  well-known  relation 
between  the  surface  and  the  mass  of  a  solid  body.  When 
a  solid  body  in  the  form  of  a  sphere  increases  in  size,  its 
mass  or  volume  increases  as  the  cube  of  the  diameter,  while 
the  surface  increases  only  as  the  square  of  the  diameter 
(Fig.  37).  The  external  surface  of  minute  animals  a  few 
millimeters  in  diameter  can  take  up  enough  food  to  supply 
the  whole  body  mass.  But  among  large  animals  this  food- 
getting  surface  is  increased  as  the  square  of  the  diameter  of 


76  ANIMAL  LIFE 

the  body,  while  the  volume  or  food-using  surface  of  the 
body  is  increased  as  the  cube  of  its  diameter.  The  food  sup- 
plying can  not  keep  pace  with  the  food  using.  Hence  it  is 
absolutely  essential  that  among  large  animals  the  food-tak- 
ing surface  be  increased  so  that  it  will  remain  in  the  same 
favorable  proportion  to  the  mass  of  the  animal  as  is  the 
case  among  the  minute  animals,  where  the  simple  external 
body  surface  is  sufficient  to  obtain  all  the  food  necessary. 
This  increase  of  surface,  without  an  accompanying  increase 
of  size  of  the  animal,  is  accomplished  by  having  the  digest- 
ing and  assimilating  surface  inside  the  body  and  by  having 
it  greatly  folded.  The  surface  of  the  alimentary  canal  is, 
after  all,  simply  a  bent-in  continuation  of  the  outer  surface 
of  the  body.  It  is  open  to  the  outside  of  the  body  by  two 
openings,  and  wholly  closed  (except  by  its  porosity)  to  the 
true  inside  of  the  body.  By  the  bending  and  coiling  of 
the  alimentary  canal,  and  by  the  repeated  folding  of  its 
inner  wall,  the  alimentary  surface  is  greatly  increased. 
The  necessity  for  this  increase  accounts  largely  for  the 
complexity  of  the  alimentary  canal. 

But  it  is  not  alone  this  necessity  for  increased  surface 
that  accounts  for  the  great  specialization  of  the  alimentary 
canal  in  such  animals  as  the  insects  and  the  vertebrates. 
The  structural  differences  in  different  portions  of  the  canal, 
resulting  in  the  differentiation  of  the  canal  into  distinct 
parts,  or  the  differentiation  of  the  whole  organ  into  distinct 
subordinate  organs,  each  with  a  special  work  or  function  to 
perform,  are  the  result  of  the  necessity  for  the  special 
manipulation  of  the  special  kinds  of  foods  taken.  Animals 
which  feed  on  other  animals  must  have  mouth  structures 
fit  for  seizing  and  rending  their  prey,  and  the  alimentary 
canal  must  be  specially  modified  for  the  digestion  of  flesh. 
Animals  which  feed  on  vegetable  substances  must  have 
special  modifications  of  the  alimentary  canal  quite  different 
from  those  of  the  carnivores.  Some  insects,  like  the  mos- 
quito, take  only  liquid  food,  the  sap  of  plants,  or  the  -blood 


FUNCTION  AND  STRUCTURE  YY 

of  animals ;  others,  like  the  weevils,  feed  on  the  hard,  dry 
substance  of  seeds  and  grains ;  others,  like  the  grasshop- 
pers and  caterpillars,  eat  green  leaves  ;  and  still  others  eat 
other  insects.  The  alimentary  canal  of  each  of  these  kinds 
of  insects  differs  more  or  less  from  that  of  the  other  kinds. 
The  specialization  of  the  alimentary  canal  depends  then 
upon  the  necessity  for  a  large  food-digesting  and  absorbing 
surface,  and  on  the  complex  treatment  of  the  food.  The 
character  of  this  specialization  in  each  case  depends  upon 
the  special  kind  or  quality  of  food  taken  by  the  animal  in 
question. 

45.  The  mutual  relation  of  function  and  structure. — The 
structure  of  an  animal  depends  upon  the  manner  in  which 
the  life  processes  or  functions  of  the  animal  are  performed. 
If  the  functions  are  performed  in  a  complex  manner,  the 
structure  of  the  body  is  complex  ;  if  the  functions  are  per- 
formed in  simple  manner,  the  body  will  be  simple  in  struc- 
ture. With  the  increase  in  degree  of  the  division  of  labor 
among  various  parts  of  the  body,  there  is  an  increase  in 
definiteness  and  extent  of  differentiation  of  structure. 
Each  part  or  organ  of  the  body  becomes  more  modified  and 
better  fitted  to  perform  its  own  special  function.  A  pecul- 
iar structural  condition  of  any  part  of  the  body,  or  of  the 
whole  body  of  any  animal,  is  not  to  be  looked  on  as  a  freak 
of  Nature,  or  as  a  wonder  or  marvel.  Such  a  structure  has 
a  significance  which  may  be  sought  for.  The  unusual 
structural  condition  is  associated  with  some  special  habit 
or  manner  of  performance  of  a  function.  Function  and 
structure  are  always  associated  in  Nature,  and  should  always 
be  associated  in  our  study  of  Nature. 


CHAPTEE  V 

THE   LIFE   CYCLE 

46.  Birth,  growth  and  development,  and  death. — Certain 
phenomena  are  familiar  to  us  as  occurring  inevitably  in  the 
life  of  every  animal.     Each  individual  is  born  in  an  imma- 
ture or  young  condition  ;  it  grows  (that  is,  it  increases  in 
size),  and  develops  (that  is,  changes  more  or  less  in  struc- 
ture), and  dies.     These  phenomena  occur  in  the  succession 
of  birth,  growth  and  development,  and  death.     But  before 
any  animal  appears  to  us  as  an  independent  individual — 
that  is,  outside  the  body  of  the  mother  and  outside  of  an 
egg  (i.  e.,  before  birth  or  hatching,  as  we  are  accustomed  to 
call  such  appearance) — it  has  already  undergone  a  longer 
or  shorter  period  of  life.     It  has  been  a  new  living  organ- 
ism hours  or  days  or  months,  perhaps,  before  its  appear- 
ance to  us.     This  period  of  life  has  been  passed  inside  an 
egg,  or  as  an  egg  or  in  the  egg  stage,  as  it  is  variously 
termed.     The  life  of  an  animal  as  a  distinct  organism  be- 
gins in  an  egg.     And  the  true  life  cycle  of  an  organism  is 
its  life  from  egg  through  birth,  growth  and  development, 
and  maturity  to  the  time  it  produces  new  organisms  in 
the  condition  of  eggs.     The  life  cycle  is  from  egg  to  egg. 
Birth  and  growth,  two  of  the  phenomena  readily  apparent 
to  us  in  the, life  of  every  animal,  are  two  phenomena  in  the 
true  life  cycle.     Death  is  a  third  inevitable  phenomenon  in 
the  life  of  each  individual,  but  it  is  not  a  part  of  the  cycle. 
It  is  something  outside. 

47.  Life  cycle  of  simplest  animals.— The  simplest  animals 
have  no  true  egg  stage,  nor  perhaps  have  they  any  true 

78 


THE  LIFE  CYCLE 


79 


-death.  The  new  Amwbce  are  from  their  beginning  like  the 
full-grown  Amo&ba,  except  as  regards  size.  And  the  old 
Amoeba  does  not  die,  because  its  whole  body  continues  to 
live,  although  in  two  parts — the  two  new  Amc&bce.  The  life 
cycle  of  the  simplest  animals  includes  birth  (usually  by 
simple  fission  of  the  body  of  the  parent),  growth,  and  some, 
but  usually  very  little,  development,  and  finally  the  repro- 
duction of  new  individuals, "not  by  the  formation  of  eggs, 
but  by  direct  division  of  the  body. 

48.  The  egg. — In  our  study  of  the  multiplication  of  ani- 
mals (Chapter  III)  we  learned  that  it  is  the  almost  univer- 


FIG.  38. — Eggs  of  different  animals  showing  variety  in  external  appearance,  a,  egg 
of  bird  ;  b,  eggs  of  toad ;  c,  egg  of  fish  ;  d,  egg  of  butterfly  ;  e,  eggs  of  katydid 
on  leaf  ;  /,egg-case  of  skate. 

sal  rule  among  many-celled  animals  that  each  individual 
begins  life  as  a  single  cell,  which  has  been  produced  by  the 


80  ANIMAL   LIFE 

fusion  of  two  germ  cells,  a  sperm  cell  from  a  male  indi- 
vidual of  the  species  and  an  egg  cell  from  a  female  indi- 
vidual of  the  species.  The  single  cell  thus  formed  is  called 
the  fertilized  egg  cell,  and  its  subsequent  development 
results  in  the  formation  of  a  new  individual  of  the  same 
species  with  its  parents.  Now,  in  the  development  of  this 
cell  into  a  new  animal,  food  is  necessary,  and  sometimes  a 
certain  amount  of  warmth.  So  with  the  fertilized  egg  cell 
there  is,  in  the  case  of  all  animals  that  lay  eggs,  a  greater 
or  less  amount  of  food  matter — food  yolk,  it  is  called — gath- 
ered about  the  germ  cell,  and  both  germ  cell  and  food  yolk 
are  inclosed  in  a  soft  or  hard  wall.  Thus  is  composed  the 
egg  as  we  know  it.  The  hen's  egg  is  as  large  as  it  is  be- 
cause of  the  great  amount  of  food  yolk  it  contains.  The 
egg  of  a  fish  as  large  as  a  hen  is  much  smaller  than  the 
hen's  egg  j  it  contains  less  food  yolk.  Eggs  (Fig.  38)  may 
vary  also  in  their  external  appearance,  because  of  the  dif- 
ferent kinds  of  membrane  or  shells  which  may  inclose  and 
protect  them.  Thus  the  frog's  eggs  are  inclosed  in  a  thin 
membrane  and  imbedded  in  a  soft,  jelly-like  substance ; 
the  skate's  egg  has  a  tough,  dark-brown  leathery  inclosing 
wall ;  the  spiral  egg  of  the  bull-head  sharks  is  leathery  and 
colored  like  the  dark-olive  seaweeds  among  which  it  lies ; 
and  a  bird's  egg  has  a  hard  shell  of  carbonate  of  lime.  But 
in  each  case  there  is  the  essential  fertilized  germ  cell ;  in 
this  the  eggs  of  hen  and  fish  and  butterfly  and  cray-fish  and 
worm  are  alike,  however  much  they  may  differ  in  size  and 
external  appearance. 

49.  Embryonic  and  post-embryonic  development. — Some 
animals  do  not  lay  eggs,  that  is  they  do  not  deposit  the  fer- 
tilized egg  cell  outside  of  the  body,  but  allow  the  develop- 
ment of  the  new  individual  to  go  on  inside  the  body  of  the 
mother  for  a  longer  or"  shorter  period.  The  mammals  and 
some  other  animals  have  this  habit.  When  such  an  ani- 
mal issues  from  the  body  of  the  mother,  it  is  said  to  be 
born.  When  the  developing  animal  issues  from  an  egg 


THE  LIFE  CYCLE 


81 


which  has  been  deposited  outside  the  body  of  the  mother, 
it  is  said  to  hatch.  The  animal  at  birth  or  at  time  of  hatch- 
ing is  not  yet  fully  developed.  Only  part  of  its  development 
or  period  of  immaturity  is  passed  within  the  egg  or  within 
the  body  of  the  mother.  That  part  of  its  life  thus  passed 
within  the  egg  or  mother's  body  is  called  the  embryonic  life 
or  embryonic  stages  of  development ;  while  that  period  of 
development  or  immaturity  from  the  time  of  birth  or  hatch- 
ing until  maturity  is  reached  is  called  the  post-embryonic 
life  or  post-embryonic  stages  of  development. 

50.  First  stages  in  development. — The  embryonic  develop- 
ment is  from  the  beginning  up  to  a  certain  point  practically 
identical  for  all  many-celled  animals — that  is,  there  are  cer- 


FIG.  39.— First  stages  in  embryonic  development  of  the  pond  snail  (Lymnoeus).  a, 
egg  cell ;  b,  first  cleavage  ;  p,  second  cleavage  ;  d,  third  cleavage  ;  e,  after  numer- 
ous cleavages ;  f,  blastula  (in  section) ;  g,  gastrula,  just  forming  (in  section) ; 
h,  gastrula,  completed  (in  section). — After  RABL. 

tain  principal  or  constant  characteristics  of  the  beginning 
development  which  are  present  in  the  development  of  all 
many-celled  animals.  The  first  stage  or  phenomenon  of 
development  is  the  simple  fission  of  the  germ  cell  into 
halves  (Fig.  39,  V).  These  two  daughter  cells  next  divide  so 
that  there  are  four  cells  (Fig.  39,  c) ;  each  of  these  divides, 
and  this  division  is  repeated  until  a  greater  or  lesser  num- 
7 


S2  ANIMAL  LIFE 

ber  (varying  with  the  various  species  or  groups  of  animals) 
of  cells  is  produced  (Fig.  39,  d).  The  phenomenon  of  re- 
peated division  of  the  germ  cell,  and  usually  the  surround- 
ing yolk,  is  called  cleavage,  and  this  cleavage  is  the  first 
stage  of  development  in  the  case  of  all  many-celled  animals. 
The  first  division  of  the  germ  cell  produces  usually  two  equal 
cells,  but  in  some  of  the  later  divisions  the  new  cells  formed 
may  not  he  equal.  In  some  animals  all  the  cleavage  cells  are 
of  equal  size ;  in  some  there  are  two  sizes  of  cells.  The  germ 
or  embryo  animal  consists  now  of  a  mass  of  few  or  many 
undifferentiated  primitive  cells  lying  together  and  usually 
forming  a  sphere  (Fig.  39,  0),  or  perhaps  separated  and  scat- 
tered through  the  food  yolk  of  the  egg.  The  next  stage  of  de- 
velopment is  this  :  the  cleavage  cells  arrange  themselves  so 
as  to  form  a  hollow  sphere  or  ball,  the  cells  lying  side  by  side 
to  form  the  outer  circumferential  wall  of  this  hollow  sphere 
(Fig.  39,/).  This  is  called  the  Uastula  or  blastoderm  stage 
of  development,  and  the  embryo  itself  is  called  the  blastula 
or  blastoderm.  This  stage  also  is  common  to  -all  the  many- 
celled  animals.  The  next  stage  in  embryonic  development 
is  formed  by  the  bending  inward  of  a  part  of  the  blasto- 
derm cell  layer,  as  shown  in  Fig.  39,  g.  This  bending  in 
may  produce  a  small  depression  or  groove  ;  but  whatever  the 
shape  or  extent  of  the  sunken-in  part  of  the  blastoderm,  it 
results  in  distinguishing  the  blastoderm  layer  into  two 
parts,  a  sunken-in  portion  called  the  endollast  and  the 
other  unmodified  portion  called  the  ectoUast.  Endo-  means 
"  within,"  and  the  cells  of  the  endoblast  often  push  so  far 
into  the  original  blastoderm  cavity  as  to  come  into  contact 
with  the  cells  of  the  ectoblast  and  thus  obliterate  this  cavity 
(Fig.  39,  h).  This  third  well-marked  stage  in  the  embry- 
onic development  is  called  the  gastrula*  stage,  and  it  also 


*  This  gastrula  stage  is  not  always  formed  by  a  bending  in  or  in- 
vagination  of  the  blastoderm,  but  in  some  animals  is  formed  by  the 
splitting  off  or  delamination  of  cells  from  a  definite  limited  region  of 


THE   HFE  CYCLE  83 

occurs  in  the  development  of  all  or  nearly  all  many-celled 
animals. 

51.  Continuity  of  development. — In  the  case  of  a  few  of 
the  simple  many-celled  animals  the  embryo  hatches — that 
is,  issues  from  the  egg  at  the  time  of  or  very  soon  after 
reaching  the  gasi^rula  stage.  In  the  higher  animals,  how- 
ever, development  goes  on  within  the  egg  or  within  the 
body  of  the  mother  until  the  embryo  becomes  a  complex 
body,  composed  of  many  various  tissues  and  organs.  Al- 
most all  the  development  may  take  place  within  the  egg, 


FIG.  40. — Honey-bee,    a,  adult  worker  ;  6,  young  or  larval  worker. 

so  that  when  the  young  animal  hatches  there  is  necessary 
little  more  than  a  rapid  growth  and  increase  of  size  to 
make  it  a  fully  developed,  mature  animal.  This  is  the  case 
with  the  birds :  a  chicken  just  hatched  has  most  of  the 
tissues  and  organs  of  a  full-grown  fowl,  and  is  simply  a 
little  hen.  But  in  the  case  of  other  animals  the  young 
hatches  from  the  egg  before  it  has  reached  such  an  ad- 
vanced stage  of  development ;  a  young  star-fish  or  young 
crab  or  young  honey-bee  (Fig.  40)  just  hatched  looks  very 
different  from  its  parent.  It  has  yet  a  great  deal  of  devel- 
opment to  undergo  before  it  reaches  the  structural  condi- 
tion of  a  fully  developed  and  fully  grown  star-fish  or  crab 
or  bee.  Thus  the  development  of  some  animals  is  almost 

the  blastoderm.  Our  knowledge  of  gastrulation  and  the  gastrula  stage 
is  yet  far  from  complete. 


84  ANIMAL  LIFE 

wholly  embryonic  development — that  is,  development  with- 
in the  egg  or  in  the  body  of  the  mother — while  the  devel- 
opment of  other  animals  is  largely  post-embryonic  or  larval 
development,  as  it  is  often  called.  There  is  no  important 
difference  between  embryonic  and  post-embryonic  develop- 
ment. The  development  is  continue  us  from  egg  cell  to 
mature  animal,  and  whether  inside  or  outside  of  an  egg  it 
goes  on  regularly  and  uninterruptedly. 

52.  Development  after  the  gastrula  stage. — The  cells  which 
compose  the  embryo  in  the  cleavage  stage  and  blastoderm 
stage,  and  even  in  the  gastrula  stage,  are  all  similar ;  there 
is  little  or  no  differentiation  shown  among  them.     But  from 
the  gastrula  stage  on  development  includes  three  important 
things :   the  gradual  differentiation  of  cells  into  various 
kinds  to  form  the  various  kinds  of  animal  tissues ;  the 
arrangement  and  grouping  of  these  cells  into  organs  and 
body  parts ;  and  finally  the  developing  of  these    organs 
and  body  parts  into  the  special  condition  characteristic  of 
the  species  of  animal  to  which  the  developing  individual 
belongs.     From  the  primitive  undifferentiated  cells  of  the 
blastoderm,  development  leads  to  the  special  cell  types  of 
muscle  tissue,  of  bone  tissue,  of  nerve  tissue ;  and  from  the 
generalized  condition  of  the  embryo  in  its  early  stages  de- 
velopment leads  to  the  specialized  condition  of  the  body  of 
the  adult  animal.     Development  is  from  the  general  to  the 
special,  as  was  said  years  ago  by  the  first  great  student  of 
development. 

53.  Divergence  of  development.— A  star-fish,  a  beetle,  a 
dove,  and  a  horse  are  all  alike  in  their  beginning-— that  is, 
the  body  of  each  is  composed  of  a  single  cell,  a  single  struc- 
tural unit.     And  they  are  all  alike,  or  very  much  alike, 
through  several  stages  of  development ;  the  body  of  each 
is  first  a  single  cell,  then  a  number  of  similar  undifferen- 
tiated cells,  and  then  a  hollow  sphere  consisting  of  a  single 
layer  of  similar  undifferentiated  cells.     But  soon  in  the 
course  of  development  the  embryos  begin  to  differ,  and  as 


THE  LIFE  CYCLE  85 

the  young  animals  get  further  and  further  along  in  the 
course  of  their  development,  they  become  more  and  more 
different  until  each  finally  reaches  its  fully  developed  ma- 
ture form,  showing  all  the  great  structural  differences  be- 
tween the  star-fish  and  the  dove,  the  beetle  and  the  horse. 
That  is,  all  animals  begin  development  alike,  but  gradually 
diverge  from  each  other  during  the  course  of  development. 
There  are  some  extremely  interesting  and  significant 
things  about  this  divergence  to  which  attention  should  be 
given.  While  all  animals  are  alike  structurally*  at  the 
beginning  of  development,  so  far  as  we  can  see,  they  do  not 
all  differ  at  the  time  of  the  first  divergence  in  development. 
This  first  divergence  is  only  to  be  noted  between  two  kinds 
of  animals  which  belong  to  different  great  groups  or  classes. 
But  two  animals  of  different  kinds,  both  belonging  to  some 
one  great  group,  do  not  show  differences  until  later  in  their 
development.  This  can  best  be  understood  by  an  example. 
All  the  butterflies  and  beetles  and  grasshoppers  and  flies 
belong  to  the  great  group  of  animals  called  Insecta,  or  in- 
sects. There  are  many  different  kinds  of  insects,  and  these 
kinds  can  be  arranged  in  subordinate  groups,  such  as  the 
Diptera,  or  flies,  the  Lepidoptera,  or  butterflies  and  moths, 
and  so  on.  But  all  have  certain  structural  characteristics 
in  common,  so  that  they  are  comprised  in  one  great  group 
or  class — the  Insecta.  Another  great  group  of  animals  is 
known  as  the  Vertebrata,  or  back-boned  animals.  The  class 
Vertebrata  includes  the  fishes,  the  batrachians,  the  reptiles, 
the  birds,  and  the  mammals,  each  composing  a  subordinate 
group,  but  all  characterized  by  the  possession  of  a  back- 

*  They  are  alike  structurally,  when  we  consider  the  cell  as  the  unit 
of  animal  structure.  That  the  egg  cells  of  different  animals  may  dif- 
fer in  their  fine  or  ultimate  structure,  seems  certain.  For  each  one  of 
these  egg  cells  is  destined  to  become  some  one  kind  of  animal,  and  no 
other ;  each  is,  indeed,  an  individual  in  simplest,  least  developed  con- 
dition of  some  one  kind  of  animal,  and  we  must  believe  that  difference 
in  kind  of  animals  depends  upon  difference  in  structure  in  the  egg  itself. 
7 


86  ANIMAL  LIFE 

.bone,  or,  more  accurately  speaking,  of  a  notochord,  a  back- 
bone-like structure.  Now,  an  insect  and  a  vertebrate  di- 
verge very  soon  in  their  development  from  each  other ;  but 
two  insects,  such  as  a  beetle  and  a  honey-bee,  or  any  two 
vertebrates,  such  as  a  frog  and  a  pigeon,  do  not  diverge 
from  each  other  so  soon.  That  is,  all  vertebrate  animals 
diverge  in  one  direction  from  the  other  great  groups,  but 
all  the  members  of  the  great  group  keep  together  for  some 
time  longer.  Then  the  subordinate  groups  of  the  Verte- 
brata,  such  as  the  fishes,  the  birds,  and  the  others  diverge, 
and  still  later  the  different  kinds  of  animals  in  each  of 
these  groups  diverge  from  each  other.  In  the  illustration 
(Fig.  41)  on  the  opposite  page  will  be  seen  pictures  of  the 
embryos  of  various  vertebrate  animals  shown  as  they  appear 
at  different  stages  or  times  in  the  course  of  development. 
The  embryos  of  a  fish,  a  salamander,  a  tortoise,  a  bird,  and 
a  mammal,  representing  the  five  principal  groups  of  the 
Vertebrata,  are  shown.  In  the  upper  row  the  embryos  are 
in  the  earliest  of  all  the  stages  figured,  and  they  are  very 
much  alike.  They  show  no  obvious  characteristics  of 
fish  or  bird.  Yet  there  are  distinctive  characteristics  of 
the  great  class  Vertebrata.  Any  of  these  embryos  could 
readily  be  distinguished  from  an  embryonic  insect  or  worm 
or  sea-urchin.  In  the  second  row  there  is  beginning  to  be 
manifest  a  divergence  among  the  different  embryos,  al- 
though it  would  still  be  a  difficult  matter  to  distinguish 
certainly  which  was  the  young  fish  and  which  the  young 
salamander,  or  which  the  young  tortoise  and  which  the 
young  bird.  In  the  bottom  row,  showing  the  animals  in  a 
later  stage  of  development,  the  divergence  has  proceeded 
so  far  that  it  is  now  plain  which  is  a  fish,  which  batrachian, 
which  reptile,  which  bird,  and  which  mammal. 

54.  The  laws  or  general  facts  of  development.— That  the 
course  of  development  of  any  animal  from  its  beginning  to 
fully  developed  adult  form  is  fixed  and  certain  is  readily 
seen.  Every  rabbit  develops  in  the  same  way  ;  every  grass- 


HI 
ffsJi 


ii  H 

|^\  Sala.  777  an  der 


ftaUtt 

FIG.  41. — Different  vertebrate  animal  in  successive  embryonic  stages.  I,  first 
or  earliest  of  the  stages  figured  ;  II,  second  of  the  stages ;  III,  third  or 
latest  of  the  stages.— After  HAECKEL. 


gg  ANIMAL  LIFE 

hopper  goes  through  the  same  developmental  changes  from 
single  egg  cell  to  the  full-grown  active  hopper  as  every 
other  grasshopper  of  the  same  kind — that  is,  development 
takes  place  according  to  certain  natural  laws,  the  laws  of 
animal  development.  These  laws  may  be  roughly  stated  as 
follows  :  All  many-celled  animals  begin  life  as  a  single  cell, 
the  fertilized  egg  cell ;  each  animal  goes  through  a  certain 
orderly  series  of  developmental  changes  which,  accom- 
panied by  growth,  leads  the  animal  to  change  from  single 
cell  to  the  many-celled,  complex  form  characteristic  of  the 
species  to  which  the  animal  belongs ;  this  development  is 
from  simple  to  complex  structural  condition ;  the  develop- 
ment is  the  same  for  all  individuals  of  one  species.  While 
all  animals  begin  development  similarly,  the  course  of  devel- 
opment in  the  different  groups  soon  diverges,  the  diver- 
gence being  of  the  nature  of  a  branching,  like  that  shown 
in  the  growth  of  a  tree.  In  the  free  tips  of  the  smallest 
branches  we  have  represented  the  various  species  of  ani- 
mals in  their  fully  developed  condition,  all  standing  clearly 
apart  from  each  other.  But  in  tracing  back  the  develop- 
ment of  any  kind  of  animal,  we  soon  come  to  a  point  where 
it  very  much  resembles  or  becomes  apparently  identical 
with  some  other  kind  of  animal,  and  going  further  back  we 
find  it  resembling  other  animals  in  their  young  condition, 
and  so  on  until  we  come  to  that  first  stage  of  development, 
that  trunk  stage,  where  all  animals  are  structurally  alike. 
To  be  sure,  any  animal  at  any  stage  in  its  existence  differs 
absolutely  from  any  other  kind  of  animal,  in  that  it  can 
develop  into  only  its  own  kind  of  animal.  There  is  some- 
thing inherent  in  each  developing  animal  that  gives  it  an 
identity  of  its  own.  Although  in  its  young  stages  it  may  be 
hardly  distinguishable  from  some  other  kind  of  animal  in 
similar  stages,  it  is  sure  to  come  out,  when  fully  developed, 
an  individual  of  the  same  kind  as  its  parents  were  or  are. 
The  young  fish  and  the  young  salamander  in  the  upper  row 
in  Fig.  41  seem  very  much  alike,  but  one  embryo  is  sure  to 


THE  LIFE  CYCLE  89 

develop  into  a  fish  and  the  other  into  a  salamander.  This 
certainty  of  an  embryo  to  become  an  individual  of  a  certain 
kind  is  called  the  law  of  heredity. 

55.  The  significance  of  the  facts  of  development.  —  The 
significance  of  the  developmental  phenomena  is  a  matter 
about  which  naturalists  have  yet  very  much  to  learn.  It  is 
believed,  however,  by  practically  all  naturalists  that  many 
of  the  various  stages  in  the  development  of  an  animal  cor- 
respond to  or  repeat  the  structural  condition  of  the  ani- 
mal's ancestors.  Naturalists  believe  that  all  backboned  or 
vertebrate  animals  are  related  to  each  other  through  being 
descended  from  a  common  ancestor,  the  first  or  oldest 
backboned  animal.  In  fact,  it  is  because  all  these  back- 
boned animals — the  fishes,  the  batrachians,  the  reptiles,  the 
birds,  and  the  mammals — have  descended  from  a  common 
ancestor  that  they  all  have  a  backbone.  It  is  believed  that 
the  descendants  of  the  first  backboned  animal  have  in  the 
course  of  many  generations  branched  off  little  by  little  from 
the  original  type  until  there  have,  come  to  exist  very  real  and 
obvious  differences  among  the  backboned  animals — differ- 
ences which  among  the  living  backboned  animals  are  familiar 
to  all  of  us.  The  course  of  development  of  an  individual  ani- 
mal is  believed  by  many  naturalists  to  be  a  very  rapid,  and 
evidently  much  condensed  and  changed,  recapitulation  of 
the  history  which  the  species  or  kind  of  animal  to  which  the' 
developing  individual  belongs  has  passed  through  in  the 
course  of  its  descent  through  a  long  series  of  gradually  chang- 
ing ancestors.  If  this  is  true,  then  we  can  readily  under- 
stand why  the  fish  and  the  salamander  and  tortoise  and 
bird  and  rabbit  are  all  so  much  alike  in  their  earlier  stages 
of  development,  and  gradually  come  to  differ  more  and 
more  as  they  pass  through  later  and  later  developmental 
stages. 

Some  naturalists  believe  that  the  ontogenetic  stages  are 
not  as  significant  in  throwing  light  upon  the  evolutionary 
history  of  the  species  as  just  indicated.  Some  think  that 


90  ANIMAL  LIFE 

when  the  earlier  stages  of  one  species  correspond  pretty 
closely  with  the  early  stages  of  another,  we  have  a  good 
basis  for  making  up  our  minds  about  relationship  between 
the  two  species.  But  it  is  certainly  not  obvious  why  we 
should  have  a  similarity  among  the  younger  stages  of  dif- 
ferent animals  and  no  correspondence  among  the  older 
stages  of  more  recent  animals  with  the  younger  stages  of 
more  ancient  ones.  But  on  the  other  hand  it  is  certainly 
true  that  a  too  specific  application  of  the  broad  generaliza- 
tion that  ontogeny  repeats  phylogeny  has  led  to  numerous 
errors  of  interpreting  genealogic  relationship. 

56.  Metamorphosis. — While  a  young  robin  when  it  hatches 
from  the  egg  or  a  young  kitten  at  birth  resembles  its  par- 
ents, a  young  star-fish  or  a  young  crab  or  a  young  butterfly 
when  hatched  does  not  at  all  resemble  its  parents.     And 
while  the  young  robin  after  hatching  becomes  a  fully  grown 
robin  simply  by  growing  larger  and  undergoing  compara- 
tively slight  developmental  changes,  the  young  star-fish  or 
young  butterfly  not  only  grows  larger,  but  undergoes  some 
very  striking  developmental  changes;  the  body  changes 
very  much  in  appearance.     Marked  changes  in  the  body  of 
an  animal  during  post-embryonic  or  larval   development 
constitute  what  is  called  metamorphic  development,  or  the 
animal  is  said  to  undergo  or  to  show  metamorphosis  in  its 
development.     Metamorphosis  is  one  of  the  most  interest- 
ing features  in  the  life  history  or  development  of  animals, 
and  it  can  be,  at  least  as  far  as  its  external  aspects  are  con- 
cerned, very  readily  observed  and  studied. 

57.  Metamorphosis  among  insects. — All  the  butterflies  and 
moths  show  metamorphosis  in  their  development.     So  do 
many  other  insects,  as  the  ants,  bees,  and  wasps,  and  all  the 
flies  and  beetles.     On  the  other  hand,  many  insects  do  not 
show  metamorphosis,  but,  like  the  birds,  are  hatched  from 
the  egg  in  a  condition  plainly  resembling  the  parents.     A 
grasshopper  (Fig.  42)  is  a  convenient  example  of  an  insect 
without  metamorphosis,  or  rather,  as  there  are,  after  all, 


THE  LIFE  CYCLE 


91 


a  few  easily  perceived  changes  in  its  post-embryonic  devel- 
opment, of  an  insect  with  an  "  incomplete  metamorpho- 
sis." The  eggs  of  grasshoppers  are  laid  in  little  packets 
of  several  score  half  an  inch  below  the  surface  of  the 
ground.  When  the  young  grasshopper  hatches  from  the 
egg  it  is  of  course  very  small,  but  it  is  plainly  recognizable 
as  a  grasshopper.  But  in  one  important  character  it  dif- 
fers from  the  adult,  and  that  is  in  its  lack  of  wings.  The 
adult  grasshopper  has  two  pairs  of  wings ;  the  just  hatched 
young  or  larval  grasshopper  has  no  wings  at  all.  The 
young  grasshopper  feeds  voraciously  and  grows  rapidly. 


FIG.  42.— Post-embryonic  development  (incomplete  metamorphosis)  of  the  Rocky 
Mountain  locust  (Melanoplus  spretus).  a,  b,  c,  d,  e,  and  f,  successive  develop- 
mental stages  from  just  hatched  to  adult  individual.— After  EMERTON. 


.In  a  few  days  it  molts,  or  casts  its  outer  skin  (not  the 
true  skin,  but  a  thin,  firm  covering  or  outer  body  wall  com- 
posed of  a  substance  called  chitin,  which  is  secreted  by  the 
cells  of  the  true  skin).  In  this  second  larval  stage  there 
can  be  seen  the  rudiments  of  four  wings,  in  the  condition 
of  tiny  wing  pads  on  the  back  of  the  middle  part  of  the 
body  (the  thorax).  Soon  the  chitinous  body  covering  is 
shed  again,  and  after  this  molt  the  wing  pads  are  mark- 
edly larger  than  before.  Still  another  molt  occurs,  with 
another  increase  in  size  of  the  developing  wings,  and  after 
a  fifth  and  last  molt  the  wings  are  fully  developed,  and 


92 


ANIMAL  LIFE 


the  grasshopper  is  no  longer  in  a  larval  or  immature  condi- 
tion, but  is  full  grown  and  adult. 

For  example  of  complete  metamorphosis  among  insects 
we  may  choose  a  butterfly,  the  large  red-brown  butterfly 


PIG.  43.— Metamorphosis  of  monarch  butterfly  (Anosia  plexippm).    a,  egg  ;  b,  larva  ; 
c,  pupa  ;  d,  imago  or  adult. 

common  in  the  United  States  and  called  the  monarch  or 
milkweed  butterfly  (Anosia  plexippus).  The  eggs  (Fig. 
43,  a)  of  this  butterfly  are  laid  on  the  leaves  of  various  kinds 
of  milkweed  (Asclepias).  The  larval  butterfly  or  butterfly 
larva  or  caterpillar  (as  the  first  young  stage  of  the  butter- 


THE   LIFE  CYCLE 


93 


flies  and  moths  is  usually  called),  which  hatches  from  the 
egg  in  three  or  four  days,  is  a  creature  bearing  little  or 
no  resemblance  to  the  beautiful  winged  adult.     The  larva 
is  worm-like,  and  instead  of  having  three  pairs  of  legs 
like  the  butterfly  it  has  eight  pairs;   it  has  biting  jaws 
in  its  mouth  with  which  it  nips  off  bits  of  the  green  milk- 
weed leaves,  instead  of  having  a  long,  slender,  sucking 
proboscis  for  drinking  flower  nectar  as  the  butterfly  has. 
The  body  of  the  crawl- 
ing    worm -like     larva 
(Fig.  43,  #)  is  greenish 
yellow    in    color,   with 
broad  rings  or  bands  of 
shining  black.     It   has 
no  wings,  of  course.    It 
eats  voraciously,  grows 
rapidly  and  molts.    But 
after  the  molting  there 
is    no    appearance    of 
rudimentary  wings;   it 
is  simply  a  larger  worm- 
like  larva.    It  continues 
to  feed  and  grow,  molt- 
ing several  times,  until 
after  the  fourth  molt  it 
appears  no  longer  as  an 
active,   crawling,   feed- 
ing, worm-like  larva,  but  as  a  quiescent,  non-feeding  pupa 
or  chrysalis  (Fig.  43,  c).     The  immature  butterfly  is  now 
greatly  contracted,  and  the  outer  chitinous  wall  is  very 
thick  and  firm.    It  is  bright  green  in  color  with  golden  dots. 
It  is  fastened  by  one  end  to  a  leaf  of  the  milkweed,  where 
it  hangs  immovable  for  from  a  few  days  to  two  weeks. 
Finally,  the  chitin  wall  of  the  chrysalis  splits,  and  there 
issues  the  full-fledged,  great,  four-winged,  red-brown  butter- 
fly (Fig.  43,  d).    Truly  this  is  a  metamorphosis,  and  a  start- 


FIQ.  44.— Metamorphosis  of  mosquito  (Culex). 
a,  larva  ;  6,  pupa. 


ANIMAL  LIFE 


ling  one.     But  we  know  that  development  in  other  animals 
is  a  gradual  and  continuous  process,  and  so  it  is  in  the 

case  of  the  butterfly. 
The  gradual  chang- 
ing is  masked  by  the 
outer  covering  of  the 
body  in  both  larva 
and  pupa.  It  is  only 
at  each  molting  or 
throwing  off  of  this 
unchanging,  unyield- 
ing chitin  armor  that 
we  perceive  how  far 
this  change  has  gone. 
The  longest  time  of 
concealment  is  that 
during  the  pupal  or 
chrysalis  stage,  and 
the  results  of  the 
changing  or  develop- 
ment when  finally  re- 
vealed by  the  split- 
ting of  the  pupal 
•case  are  hence  the 
most  striking. 

58.  Metamorphosis  of  the  toad. — Metamorphosis  is  found 
in  the  development  of  numerous  other  animals,  as  well  as 
among  the  insects.  Certain  cases  are  familiar  to  all — the 
metamorphosis  of  the  frogs  and  toads  (Fig.  46).  The  eggs 
of  the  toad  are  arranged  in  long  strings  or  ribbons  in  a 
transparent  jelly-like  substance.  These  jelly  ribbons  with 
the  small,  black,  bead-like  eggs  in  them  are  wound  around 
the  stems  of  submerged  plants  or  sticks  near  the  shores  of 
the  pond.  From  each  egg  hatches  a  tiny,  wriggling  tad- 
pole, differing  nearly  as  much  from  a  full-grown  toad  as 
a  caterpillar  differs  from  a  butterfly.  The  tadpoles  feed  on 


FIG.  45. — Larva  of  a  butterfly  just  changing  into 
pupa  (making  last  larval  molt).  Photograph 
from  Nature. 


THE  LIFE  CYCLE 


95 


the  microscopic  plants  to  be  found  in  the  water,  and  swim 
easily  about  by  means  of  the  long  tail.  The  very  young 
tadpoles  remain  underneath  the  surface  of  the  water  all  the 
time,  breathing  the  air  which  is  mixed  with  water  by  means 
of  gills.  But  as  they  become  older  and  larger  they  come 
often  to  the  surface  of  the  water.  Lungs  are  developing 
inside  the  body,  and  the  tadpole  is  beginning  to  breathe  as 
a  land  animal,  although  it  still  breathes  partly  by  means  of 
gills,  that  is,  as  an  aquatic  animal.  Soon  it  is  apparent  that 
although  the  tadpole  is  steadily  and  rapidly  growing  larger, 
its  tail  is  growing  shorter  and  smaller  instead  of  larger.  At 
the  same  time,  fore  and  hind  legs  bud  out  and  rapidly  take 


FIG.  46.— Metamorphosis  of  the  toad  (partly  after  GASE).    At  left  the  strings  of  eggs, 
in  water  the  various  tadpole  or  larval  stages,  and  on  bank  the  adult  toads. 

form  and  become  functional.  By  the  time  that  the  tail 
gets  very  short,  indeed,  the  young  toad  is  ready  to  leave  the 
water  and  live  as  a  land  animal.  On  land  the  toad  lives,  as 


96 


ANIMAL  LIFE 


we  know,  on  insects  and  snails  and  worms.  The  metamor- 
phosis of  the  toad  is  not  so  striking  as  that  of  the  butter- 
fly, but  if  the  tadpole  were  inclosed  in  an  unchanging 
opaque  body  wall  while  it  was  losing  its  tail  and  getting  its 
legs,  and  this  wall  were  to  be  shed  after  these  changes  were 
made,  would  not  the  metamorphosis  be  nearly  as  extraordi- 


Fie.  47.— Metamorphosis  of  sea- 
urchin.  Upper  figure  the  adult, 
lower  figure  the  pluteus  larva. 


nary  as  in  the  case  of 
the  butterfly?  But  in 
the  metamorphosis  of 
the  toad  we  can  see  the 
gradual  and  continuous 
character  of  the  change. 

59.  Metamorphosis  among  other  animals. — Many  other 
animals,  besides  insects  and  frogs  and  toads,  undergo  meta- 
morphosis. The  just-hatched  sea-urchin  does  not  resemble 
a  fully  developed  sea-urchin  at  all.  It  is  a  minute  worm- 
like  creature,  provided  with  cilia  or  vibratile  hairs,  by -means 
of  which  it  swims  freely  about.  It  changes  next  into  a  curi- 
ous bootjack-shaped  body  called  the  pluteus  stage  (Fig,  47). 
In  the  pluteus  a  skeleton  of  lime  is  formed,  and  the  final 
true  sea-urchin  body  begins  to  appear  inside  the  pluteus, 


THE  LIFE  CYCLE 


97 


developing  and  growing  by  using  up  the  body  substance  of 
the  pluteus.  Star-fishes,  which  are  closely  related  to  sea- 
urchins,  show  a  simi- 
lar metamorphosis, 
except  that  there  is 
no  pluteus  stage,  the 
true  star-fish-shaped 
body  forming,  with- 
in and  at  the  expense 
of  the  first  larval 
stage,  the  ciliated 
free-swimming  stage. 
A  young  crab  just 
issued  from  the  egg 
(Fig.  48)  is  a  very 
different  appearing 
creature  from  the 
adult  or  fully  devel- 
oped crab.  The  body 
of  the  crab  in  its 
first  larval  stage  is 

Composed  of  a  short,    FlG'  ^-Metamorphosis  of  the  crab. 

globular  portion,  fur- 
nished with  conspicuous  long  spines  and  a  relatively  long, 
jointed  tail.  This  is  called  the  zoea  stage.  The  zoe'a 
changes  into  a  stage  called  the  megalops,  which  has  many 
characteristics  of  the  adult  crab  condition,  but  differs  espe- 
cially from  it  in  the  possession  of  a  long,  segmented  tail, 
and  in  having  the  front  half  of  the  body  longer  than  wide. 
The  crab  in  the  megalops  stage  looks  very  much  like  a 
tiny  lobster  or  shrimp.  The  tail  soon  disappears  and  the 
body  widens,  and  the  final  stage  is  reached. 

In  many  families  of  fishes  the  changes  which  take  place 

in  the  course  of  the-li^e  cycle  are  almost  as  great  as  in  the 

case  of  the  insect  or  the  toad.     In  the  lady-fish  (Albula 

vulpes)  the  very  young  (Fig.  49)  are  ribbon-like  in  form, 

8 


the  zoe'a 
b,  the  megalops ;  c,  the  adult. 


98 


ANIMAL  LIFE 


with  small  heads  and  very  loose  texture  of  the  tissues,  the 
body  substance  being  jelly-like  and  transparent.  As  the  fish 
grows  older  the  body  becomes  more  compact,  and  therefore 


rf/7777?f~/T77ff77T:: .  '/  •  •  •  .'/.•'  •'  .•'  ••//••/  :  l .;'  • 


FIG.  49. — Stages  in  the  post-embryonic  development  of  the  lady-fish  (Albula  vulpes), 
showing  metamorphosis.  —After  C.  H.  GILBERT. 

shorter  and  slimmer.    After,  shrinking  to  the  texture  of  an 
ordinary  fish,  its  growth  in  size  begins  normally,  although 


THE   LIFE  CYCLE 


99 


it  has  steadily  increased  in  actual  weight.  Many  herring, 
eels,  and  other  soft-bodied  fishes  pass  through  stages  simi- 
lar to  those  seen  in  the  lady-fish.  Another  type  of  devel- 
opment is  illustrated  in  the  sword-fish.  The  young  has  a 
bony  head,  bristling  with  spines.  As  it  grows  older  the 
spines  disappear,  the  skin  grows  smoother,  and,  finally,  the 
bones  of  the  upper  jaw  grow  together,  forming  a  prolonged 
sword,  the  teeth  are  lost  and  the  fins  become  greatly  modi- 
fied. Fig.  50  shows  three  of  these  stages  of  growth.  The 


a 


FIG.  50.— Three  stages  in  the  development  of  the  sword-fish  (Xiphias  gladiti-s). 
a,  very  young  ;  b,  older ;  c,  adult.— Partly  after  LUTKBN. 

flounder  or  flat-fish  (Fig.  51)  when  full  grown  lies  flat  on 
one  side  when  swimming  or  when  resting  in  the  sand  on 
the  bottom  of  the  sea.  The  eyes  are  both  on  the  upper 
side  of  the  body,  and  the  lower  side  is  blind  and  colorless. 
When  the  flounder  is  hatched  it  is  a  transparent  fish,  broad 
and  flat,  swimming  vertically  in  the  water,  with  an  eye  on 
each  side.  As  its  development  (Fig.  52)  goes  on  it  rests 
itself  obliquely  on  the  bottom,  the  eye  of  the  lower  side 
turns  upward,  and  as  growth  proceeds  it  passes  gradually 


100 


ANIMAL   LIFE 


around  the  forehead,  its  socket  moving  with  it,  until  both 
eyes  and  sockets  are  transferred  by  twisting  of  the  skull  to 


PIG.  51.— The  wide-eyed  flounder  (Platophrys  lunatus).   Adult,  showing  both  eyes  on 
upper  side  of  head. 

the  upper  side.  In  some  related  forms  or  soles  the  small 
eye  passes  through  the  head  and  not  around  it,  appearing 
finally  in  the  same  socket  with  the  other  eye. 

Thus  in  almost  all  the  great  groups  of  animals  we  find 
certain  kinds  which  show  metamorphosis  in  their  post- 
embryonic  development.  But  metamorphosis  is  simply 
development;  its  striking  and  extraordinary  features  are 
usually  due  to  the  fact  that  the  orderly,  gradual  course  of 
the  development  is  revealed  to  us  only  occasionally,  with 
the  result  of  giving  the  impression  that  the  development  is 
proceeding  by  leaps  and  bounds  from  one  strange  stage  to 


FIG.  52.— Development  of  a  flounder  (after  EMERY).    The  eyes  in  the  young  flounder 
are  arranged  normally,  one  on  each  side  of  head. 

another.     If  metamorphosis  is  carefully  studied  it  loses  its 
aspect  of  marvel,  although  never  its  great  interest. 


THE  LIFE  CYCLED 


60.  Duration  of  life.  —  After  an  animal  has  completed  its 
development  it  has  but  one  thing  to  do  to  complete  its  life 
cycle,  and  that  is  the  production  of  offspring.  When  it 
has  laid  eggs  or  given  birth  to  young,  it  has  insured  the 
beginning  of  a  new  life  cycle.  Does  it  now  die  ?  Is  the 
business  of  its  life  accomplished  ?  There  are  many  animals 
which  die  immediately  or  very  soon  after  laying  eggs.  The 
May-flies  —  ephemeral  insects  which  issue  as  winged  adults 
from  ponds  or  lakes  in  which 
they  have  spent  from  one  to 
three  years  as  aquatic  crawl- 
ing or  swimming  larvae,  flutter 
about  for  an  evening,  mate, 
drop  their  packets  of  fertil- 
ized eggs  into  the  water,  and 
die  before  the  sunrise  —  are 
extreme  examples  of  the  nu- 
merous kinds  of  animals 
whose  adult  life  lasts  only  long 
enough  for  mating  and  egg- 
laying.  But  elephants  live  for 
two  hundred  years.  Whales 
probably  live  longer.  A  horse 
lives  about  thirty  years,  and  so 
may  a  cat  or  toad.  A  sea- 
anemone,  which  was  kept  in  an  aquarium,  lived  sixty-six 
years.  Cray-fishes  may  live  twenty  years.  A  queen  bee 
was  kept  in  captivity  for  fifteen  years.  Most  birds  have 
long  lives  —  the  small  song  birds  from  eight  to  eighteen 
years,  and  the  great  eagles  and  vultures  up  to  a  hundred 
years  or  more.  On  the  other  hand,  among  all  the  thou- 
sands of  species  of  insects,  the  individuals  of  very  few  in- 
deed live  more  than  a  year  ;  the  adult  life  of  most  insects 
being  but  a  few  days  or  weeks,  or  at  best  months.  Even 
among  the  higher  animals,  some  are  very  short-lived. 
In  Japan  is  a  small  fish  (Solaux)  which  probably  lives 


**  • 


V  *:  t  .ct  :  \  ^.v  ANIMAL  LIFE 

but  a  year,  ascending  the  rivers  in  numbers  when  young  in 
the  spring,  the  whole  mass  of  individuals  dying  in  the  fall 
after  spawning. 

Naturalists  have  sought  to  discover  the  reason  for  these 
extraordinary  differences  in  the  duration  of  life  of  different 
animals,  and  while  it  can  not  be  said  that  the  reason  or 
reasons  are  wholly  known,  yet  the  probability  is  strong  that 
the  duration  of  life  is  closely  connected  with,  or  dependent 
upon,  the  conditions  attending  the  production  of  offspring. 
It  is  not  sufficient,  as  we  have  learned  from  our  study  of 
the  multiplication  of  animals  (Chapter  III),  that  an  adult 
animal  shall  produce  simply  a  single  new  individual  of  its 
kind,  or  even  only  a  few.  It  must  produce  many,  or  if  it 
produces  comparatively  few  it  must  devote  great  care  to 
the  rearing  of  these  few,  if  the  perpetuation  of  the  species 
is  to  be  insured.  Now,  almost  all  long-lived  animals  are 
species  which  produce  but  few  offspring  at  a  time,  and 
reproduce  only  at  long  intervals,  while  most  short-lived  ani- 
mals produce  a  great  many  eggs,  and  these  all  at  one  time. 
Birds  are  long-lived  animals;  as  we  know,  most  of  them 
lay  eggs  but  once  a  year,  and  lay  only  a  few  eggs  each  time. 
Many  of  the  sea  birds  which  swarm  in  countless  numbers 
on  the  rocky  ocean  islets  and  great  sea  cliffs  lay  only  a 
single  egg  once  each  year.  And  these  birds,  the  guillemots 
and  murres  and  auks,  are  especially  long-lived.  Insects,  on 
the  contrary,  usually  produce  many  eggs,  and  all  of  them 
in  a  short  time.  The  May-fly,  with  its  one  evening's  lifetime, 
lets  fall  from  its  body  two  packets  of  eggs  and  then  dies. 
Thus  the  shortening  of  the  period  of  reproduction  with  the 
production  of  a  great  many  offspring  seem  to  be  always 
associated  with  a  short  adult  lifetime ;  while  a  long  period 
of  reproduction  with  the  production  of  few  offspring  at  a 
time  and  care  of  the  offspring  are  associated  with  a  long 
adult  lifetime. 

There  seems  also  to  be  some  relation  between  the  size 
of  animals  and  the  length  of  life.  As  a  general  rule, 


THE  LIFE  CYCLE  103 

large  animals  are  long-lived  and  small  animals  have  short 
lives. 

61.  Death. — At  the  end  comes  death.  After  the  animal 
has  completed  its  life  cycle,  after  it  has  done  its  share  toward 
insuring  the  perpetuation  of  its  species,  it  dies.  It  may 
meet  a  violent  death,  may  be  killed  by  accident  or  by  ene- 
mies, before  the  life  cycle  is  completed.  And  this  is  the 
fate  of  the  vast  majority  of  animals  which  are  born  or 
hatched.  Or  death  may  come  before  the  time  for  birth  or 
hatching.  Of  the  millions  of  eggs  laid  by  a  fish,  each  egg 
a  new  fish  in  simplest  stage  of  development,  how  many  or 
rather  how  few  come  to  maturity,  how  few  complete  the 
cycle  of  life ! 

Of  death  we  know  the  essential  meaning.  Life  ceases 
and  can  never  be  renewed  in  the  body  of  the  dead  animal. 
It  is  important  that  we  include  the  words  "  can  never  be 
renewed,"  for  to  say  simply  that  "  life  ceases,"  that  is,  that 
the  performance  of  the  life  processes  or  functions  ceases, 
is  not  really  death.  It  is  easy  to  distinguish  in  most  cases 
between  life  and  death,  between  a  live  animal  and  a  dead 
one,  yet  there  are  cases  of  apparent  death  or  a  semblance  of 
death  which  are  very  puzzling.  The  test  of  life  is  usually 
taken  to  be  the  performance  of  life  functions,  the  assimila- 
tion of  food  and  excretion  of  waste,  the  breathing  in  of  oxy- 
gen, and  breathing  out  of  carbonic-acid  gas,  movement, 
feeling,  etc.  But  some  animals  can  actually  suspend  all 
of  these  functions,  or  at  least  reduce  them  to  such  a  mini- 
mum that  they  can  not  be  perceived  by  the  strictest  exami- 
nation, and  yet  not  be  dead.  That  is,  they  can  renew 
again  the  performance  of  the  life  processes.  Bears  and 
some  other  animals,  among  them  many  insects,  spend  the 
winter  in  a  state  of  death-like  sleep.  Perhaps  it  is  but  sleep ; 
and  yet  hibernating  insects  can  be  frozen  solid  and  remain 
frozen  for  weeks  and  months,  and  still  retain  the  power  of 
actively  living  again  in  the  following  spring.  Even  more 
remarkable  is  the  case  of  certain  minute  animals  called  Ro- 


104  ANIMAL  LIFE 

tatoria  and  of  others  called  Tardigrada,  or  bear-animalcules. 
These  bear-animalcules  live  in  water.  If  the  water  dries 
up,  the  animalcules  dry  up  too ;  they  shrivel  up  into  form- 
less little  masses  and  become  desiccated.  They  are  thus 
simply  dried-up  bits  of  organic  matter;  they  are  organic 
dust.  Now,  if  after  a  long  time — years  even — one  of  these 
organic  dust  particles,  one  of  these  dried-up  bear-animal- 
cules is  put  into  water,  a  strange  thing  happens.  The  body 
swells  and  stretches  out,  the  skin  becomes  smooth  instead 
of  all  wrinkled  and  folded,  and  the  legs  appear  in  normal 
shape.  The  body  is  again  as  it  was  years  before,  and  after 
a  quarter  of  an  hour  to  several  hours  (depending  on  the 
length  of  time  the  animal  has  lain  dormant  and  dried)  slow 
movements  of  the  body  parts  begin,  and  soon  the  animal- 
cule crawls  about,  begins  again  its  life  where  it  had  been 
interrupted.  Various  other  small  animals,  such  as  vinegar 
eels  and  certain  Protozoa,  show  similar  powers.  Certainly 
here  is  an  interesting  problem  in  life  and  death. 

When  death  comes  to  one  of  the  animals  with  which 
we  are  familiar,  we  are  accustomed  to  think  of  its  coming 
to  the  whole  body  at  some  exact  moment  of  time.  As  we 
stand  beside  a  pet  which  has  been  fatally  injured,  we  wait 
until  suddenly  we  say,  "  It  is  dead."  As  a  matter  of  fact, 
it  is  difficult  to  say  when  death  occurs.  Long  after  the 
heart  ceases  to  beat,  other  organs  of  the  body  are  alive — 
that  is,  are  able  to  perform  their  special  functions.  The 
muscles  can  contract  for  minutes  or  hours  (for  a  short  time 
in  warm-blooded,  for  a  long  time  in  cold-blooded  animals) 
after  the  animal  ceases  to  breathe  and  its  heart  to  beat. 
Even  longer  live  certain  cells  of  the  body,  especially  the 
amoeboid  white  blood-corpuscles.  These  cells,  very  like 
the  Amoeba  in  character,  live  for  days  after  the  animal  is, 
as  we  say,  dead.  The  cells  which  line  the  tracheal  tube 
leading  to  the  lungs  bear  cilia  or  fine  hairs  which  they 
wave  back  and  forth.  They  continue  this  movement  for 
days  after  the  heart  has  ceased  beating.  Among  cold- 


THE  LIFE  CYCLE 


105 


blooded  animals,  like  snakes  and  turtles,  complete  cessa- 
tion of  life  functions  comes  very  slowly,  even  after  the 
body  has  been  literally  cut  to  pieces. 

Thus  it  is  essential  in  defining  death  to  speak  of  a 
complete  and  permanent  cessation  of  the  performance  of 
the  life  processes. 


A  grasshopper  (Melanoplus  differentialis)  killed  by  disease  caused  by  a 
parasitic  fungus.    On  golden-rod. 


CHAPTER  VI 

THE   PRIMARY   CONDITIONS   OF   ANIMAL   LIFE 

62.  Primary   conditions  and  special  conditions. — Certain 
primary  conditions  are  necessary  for  the  existence  of  all 
animals.    We  know  that  fishes  can  not  live  very  long  out 
of  water,  and  that  birds  can  not  live  in  water.     These, 
however,  are  special  conditions  which  depend  on  the  spe- 
cial structure  and  habits  of  these  two  particular  kinds  of 
backboned  animals.     But  the  necessity  of  a  constant  and 
sufficient  supply  of  air  is  a  necessity  common  to  both  ;  it  is 
one  of  the  primary  conditions  of  their  life.     All  animals 
must  have  air.     Similarly  both  fishes  and  birds,  and  all 
other  animals  as  well,  must  have  food.     This  is  another  one 
of  the  primary  conditions  of  animal  life.     That  backboned 
animals  must  find  somehow  a  supply  of  salts  or  compounds 
of  lime  to  form  into  bones  is  a  special  condition  peculiar 
to  these  animals.     Other  animals  having  shells  or  teeth 
composed  of  carbonate  or  phosphate  of  lime  are  subject  to 
the  same  special  demand,  but  many  animals  have  no  hard 
parts,  and  therefore  need  no  lime. 

63.  Food. — All  the  higher  plants,  those  that  are  green 
(chlorophyll-bearing),  can  make  their  living  substance  out 
of  inorganic  matter  alone — that  is,  use  inorganic  substances 
as  food.     But  animals  can  not  do  this.     They  must  have 
already  formed  organic  matter  for  food.     This  organic  mat- 
ter may  be  the  living  or  dead  tissues  of  plants,  or  the  living 
or  dead  tissues  of  animals.     For  the  life  of  animals  it  is 
necessary  that  other  organisms  live,  or  have  lived.     It  is 
this  need  which  primarily  distinguishes  an  animal  from  a 


THE   PRIMARY  CONDITIONS  OF  ANIMAL  LIFE     107 

plant.  Animals  can  not  exist  without  plants.  The  plants 
furnish  all  animals  with  food,  either  directly  or  indirectly. 
The  amount  of  food  and  the  kinds  of  food  required  by 
various  kinds  of  animals  are  special  conditions  depending 
on  the  size,  the  degree  of  activity,  the  structural  character 
of  the  body,  etc.,  of  the  animal  in  question.  Those  which 
do  the  most  need  most.  Those  with  warmest  blood,  great- 
est activity,  and  most  rapid  change  of  tissues  are  most 
dependent  on  abundance,  regularity,  and  fitness  of  their 
food.  As  we  well  know,  an  animal  can  live  for  a  longer  or 
shorter  time  without  food.  Men  have  fasted  for  a  month, 
or  even  two  months.  Among  cold-blooded  animals,  like  the 
reptiles,  the  general  habit  of  food  taking  is  that  of  an  occa- 
sional gorging,  succeeded  by  a  long  period  of  abstinence. 
Many  of  the  lower  animals  can  go  without  food  for  surpris- 
ingly long  periods  without  loss  of  life.  But  the  continued 
lack  of  food  results  inevitably  in  death.  Any  animal  may 
be  starved  in  time. 

If  water  be  held  not  to  be  included  in  the  general  con- 
ception of  food,  then  special  mention  must  be  made  of  the 
necessity  of  water  as  one  of  the  primary  conditions  of  ani- 
mal life.  Protoplasm,  the  basis  of  life,  is  a  fluid,  although 
thick  and  viscous.  To  be  fluid  its  components  must  be 
dissolved  or  suspended  in  water.  In  fact,  all  the  truly 
living  substance  in  an  animal's  body  contains  water.  The 
water  necessary  for  the  animal  may  be  derived  from  the 
other  food,  all  of  which  contains  water  in  greater  or 
less  quantity,  or  may  be  taken  apart  from  the  other 
food,  by  drinking  or  by  absorption  through  the  skin. 
Sheep  are  seldom  seen  to  drink,  for  they  find  .almost 
enough  water  in  their  green  food.  Fur  seals  never  drink, 
for  they  absorb  the  water  needed  through  pores  in  the 
skin. 

64.  Oxygen. — Animals  must  have  air  in  order  to  live, 
but  the  essential  element  of  the  air  which  they  need  is  its 
oxygen.  For  the  metabolism  of  the  body,  for  the  chemical 


108  ANIMAL  LIFE 

changes  which  take  place  in  the  body  of  every  living  ani- 
mal, a  supply  of  oxygen  is  required.  This  oxygen  is  de- 
rived directly  or  indirectly  from  the  air.  The  atmosphere 
of  the  earth  is  composed  of  79.02  parts  of  nitrogen  (includ- 
ing argon),  .03  parts  of  carbonic  acid,  and  20.95  parts  of 
oxygen.  Thus  all  the  animals  which  live  on  land  are  en- 
veloped by  a  substance  containing  nearly  21  per  cent  of 
oxygen.  But  animals  can  live  in  an  atmosphere  containing 
much  less  oxygen.  Certain  mammals,  experimented  on, 
lived  without  difficulty  in  an  atmosphere  containing  only 
14  per  cent  of  oxygen  ;  when  the  oxygen  was  reduced  to  7 
per  cent  serious  disturbances  were  caused  in  the  animal's 
condition,  and  death  by  suffocation  ensued  when  3  per 
cent  of  oxygen  was  left  in  the  atmosphere.  Animals  which 
live  in  water  get  their  oxygen,  not  from  the  water  itself 
(water  being  composed  of  hydrogen  and  oxygen),  but  from 
air  which  is  mechanically  mixed  with  the  water.  Fishes 
breathe  the  air  which  is  mixed  with  or  dissolved  in  the 
water.  This  scanty  supply  therefore  constitutes  their  at- 
mosphere, for  in  water  from  which  all  air  is  excluded  no 
animal  can  breathe.  Whatever  the  habits  of  life  of  the 
animal,  whether  it  lives  on  the  land,  in  the  ground,  or  in 
the  water,  it  must  have  oxygen  or  die. 

6,5.  Temperature,  pressure,  and  other  conditions. — Some 
physiologists  include  among  the  primary  or  essential  gen- 
eral conditions  of  animal  life  such  conditions  as  favorable 
temperature  and  favorable  pressure.  It  is  known  from  ob- 
servation and  experiment  that  animals  die  when  a  too  low 
or  a  too  high  temperature  prevails.  The  minimum  or 
maximum  of  temperature  between  which  limits  an  animal 
can  live  varies  much  among  different  kinds  of  animals.  It 
is  familiar  knowledge  that  many  kinds  of  animals  can  be 
frozen  and  yet  not  be  killed.  Insects  and  other  small  ani- 
mals may  lie  frozen  through  a  winter  and  resume  active 
life  again  in  the  spring.  An  experimenter  kept  certain 
fish  frozen  in  blocks  of  ice  at  a  temperature  of  —15°  C. 


THE  PRIMARY  CONDITIONS  OF  ANIMAL  LIFE    109 

for  some  time  and  then  gradually  thawed  them  out  un- 
hurt. Only  very  hardy  kinds  adapted  to  the  cold  would, 
however,  survive  such  treatment.  There  is  no  doubt  that 
every  part  of  the  body,  all  of  the  living  substance,  of  these 
fish  was  frozen,  for  specimens  at  this  temperature  could  be 
broken  and  pounded  up  into  fine  ice  powder.  But  a  tem- 
perature of  —20°  C.  killed  the  fish.  Frogs  lived  after  being 
kept  at  a  temperature  of  —28°  C.,  centipeds  at  —50°  C.,  and 
certain  snails  endured  a  temperature  of  — 120°  C.  without 
dying.  At  the  other  extreme,  instances  are  known  of  ani- 
mals living  in  water  (hot  springs  or  water  gradually  heated 
with  the  organisms  in  it)  of  a  temperature  as  high  as  50°  C. 
Experiments  with  Amcebce  show  that  these  simplest  animals 
contract  and  cease  active  motion  at  35°  C.,  but  are  not  killed 
until  a  temperature  of  40°  to  45°  C.  is  reached.  The  little 
fish  called  blob  or  miller's  thumb  ( Coitus  ictalops)  has  been 
seen  lying  boiled  in  the  bottom  of  the  hot  springs  in  the 
Yellowstone  Park ;  but  it  must  have  entered  these  springs 
through  streams  of  a  temperature  little  below  the  boiling 
point. 

The  pressure  or  weight  of  the  atmosphere  on  the  sur- 
face of  the  earth  is  nearly  fifteen  pounds  on  each  square 
inch.  This  pressure  is  exerted  equally  in  all  directions,  so 
that  an  object  on  the  earth's  surface  sustains  a  pressure  on 
each  square  inch  of  its  surface  exposed  to  the  air  of  fifteen 
pounds.  Thus  all  animals  living  on  the  earth's  surface  or 
near  it  live  under  this  pressure,  and  know  no  other  condi- 
tion. For  this  reason  they  do  not  notice  it.  The  animals 
that  live  in  water,  however,  sustain  a  much  greater  pres- 
sure, this  pressure  increasing  with  the  depth.  Certain 
ocean  fishes  live  habitually  at  great  depths,  as  two  to  five 
miles,  where  the  pressure  is  equivalent  to  that  of  many 
hundred  atmospheres.  If  these  fishes  are  brought  to  the 
surface  their  eyes  bulge  out  fearfully,  being  pushed  out 
through  reduced  expansion ;  their  scales  fall  off  because  of 
the  great  expansion  of  the  skin,  and  the  stomach  is  pushed 


110  ANIMAL  LIFE 

out  from  the  mouth  till  it  is  wrong  side  out.  Indeed,  the 
bodies  sometimes  burst.  Their  bodies  are  accustomed  to 
this  great  pressure,  and  when  this  outside  pressure  is  sud- 
denly removed  the  body  may  be  bursted.  Sometimes 
such  a  fish  is  raised  from  its  proper  level  by  a  struggle 
with  its  prey,  when  both  captor  and  victim  may  be  de- 
stroyed by  the  expansion  of  the  body.  Some  fishes  die  on 
being  taken  out  of  water  through  the  swelling  of  the  air 
bladder  and  the  bursting  of  its  blood-vessels.  If  an  animal 
which  lives  normally  on  the  surface  of  the  earth  is  taken 
up  a  very  high  mountain  or  is  carried  up  in  a  balloon  to  a 
great  altitude  where  the  pressure  of  the  atmosphere  is 
much  less  than  it  is  at  the  earth's  surface,  serious  conse- 
quences may  ensue,  and  if  too  high  an  altitude  is  reached 
death  occurs.  This  death  may  be  in  part  due  to  the  diffi- 
culty in  breathing  in  sufficient  oxygen  to  maintain  life,  but 
it  is  probably  chiefly  due  to  disturbances  caused  by  the 
removal  of  the  pressure  to  which  the  body  is  accustomed 
and  is  structurally  adapted  to  withstand.  A  famous  bal- 
loon ascension  was  made  in  Paris  in  1875  by  three  men. 
After  the  balloon  had  reached  a  height  of  nearly  24,000 
feet  (almost  five  miles)  the  men  began  to  lose  conscious- 
ness. On  the  sinking  of  the  balloon  to  about  20,000  feet 
the  men  regained  consciousness  again  and  threw  out  bal- 
last so  that  the  balloon  rose  to  a  height  of  over  25,000  feet. 
This  time  all  three  became  wholly  unconscious,  and  on  the 
balloon  sinking  again  only  one  regained  consciousness. 
The  other  two  died  in  the  foolhardy  experiment.  All  liv- 
ing animals  are  accustomed  to  live  under  a  certain  pres- 
sure, and  there  are  evidently  limits  of  maximum  or  mini- 
mum pressure  beyond  which  no  animal  at  present  existing 
can  go  and  remain  alive. 

But  in  the  case  both  of  temperature  and  pressure  con- 
ditions it  is  easy  to  conceive  that  animals  might  exist  which 
could  live  under  temperature  and  pressure  conditions  not 
included  between  the  minimum  and  maximum  limits  of  each 


THE  PRIMARY  CONDITIONS  OF  ANIMAL  LIFE 

as  determined  by  animals  so  existing.  But  it  is  impossible 
to  conceive  of  animals  which  could  live  without  oxygen  or 
without  organic  food.  The  necessities  of  oxygen  and  organic 
food  (and  water)  are  the  primary  or  essential  conditions 
for  the  existence  of  any  animals. 

Of  course,  we  might  include  such  conditions,  among 
the  primary  conditions,  as  the  light  and  heat  of  the  sun, 
the  action  of  gravitation,  and  olher  physical  conditions 
without  which  existence  or  life  of  any  kind  would  be  im- 
possible on  this  earth.  But  we  here  consider  by  "  primary 
conditions  of  animal  life  "  rather  those  necessities  of  living 
animals  as  opposed  to  the  necessities  of  living  plants. 
Neither  animals  nor  plants  could  exist  without  the  sun, 
whence  they  derive  directly  or  indirectly  all  their  energy. 

66.  Difference  between  animals  and  plants. — It  is  easy  to 
distinguish  between  the  animal  and  plant  when  a  butterfly 
is  fluttering  about  a  blossoming  cherry  tree  or  a  cow  feed- 
ing in  a  field  of  clover.  It  is  not  so  easy,  if  it  is,  indeed, 
possible,  to  say  which  is  plant  and  which  is  animal  when 
the  simplest  plants  are  compared  with  the  simplest  ani- 
mals. It  is  almost  impossible  to  so  define  animals  as  to 
distinguish  all  of  them  from  all  plants,  or  so  to  define 
plants  as  to  distinguish  all  of  them  from  all  animals. 
While  most  animals  have  the  power  of  locomotion,  some, 
like  the  sponges  and  polyps  and  barnacles  and  numerous 
parasites,  are  fixed.  While  most  plants  are  fixed,  some  of 
the  low  aquatic  forms  have  the  po  sver  of  spontaneous  loco- 
motion, and  all  plants  have  some  power  of  motion,  as  espe- 
cially exemplified  in  the  revolution  of  the  apex  of  the 
growing  stem  and  root,  and  the  spiral  twisting  of  tendrils, 
and  in  the  sudden  closing  of  the  leaves  of  the  sensitive 
plant  when  touched.  Among  the  green  or  chlorophyll- 
bearing  plants  the  food  consists  chiefly  of  inorganic  sub- 
stances, especially  of  carbon  which  is  taken  from  the  car- 
bonic-acid gas  in  the  atmosphere,  and  of  water.  But  some 
green-leaved  plants  feed  also  in  part  on  organic  food. 


112  ANIMAL  LIFE 

Such  are  the  pitcher-plants  and  sun-dews,  and  Venus-fly- 
traps, which  catch  insects  and  use  them  for  food  nutrition. 
But  there  are  many  plants,  the  fungi,  which  are  not  green 
— that  is,  which  do  not  possess  chlorophyll,  the  substance 
on  which  seems  to  depend  the  power  to  make  organic 
matter  out  of  inorganic  substances.  These  plants  feed  on 
organic  matter  as  animals  do.  The  cells  of  plants  (in  their 
young  stages,  at  least)  have  a  wall  composed  of  a  peculiar 
carbohydrate  substance  called  cellulose,  and  this  cellulose 
was  for  a  long  time  believed  not  to  occur  in  the  body  of 
animals.  But  now 'it  is  known  that  certain  sea-squirts 
(Tunicata)  possess  cellulose.  It  is  impossible  to  find  any 
set  of  characteristics,  or  even  any  one  characteristic,  which 
is  possessed  only  by  plants  or  only  by  animals.  But  nearly 
all  of  the  many-celled  plants  and  animals  may  be  easily 
distinguished  by  their  general  characteristics.  The  power 
of  breaking  up  carbonic-acid  gas  into  carbon  and  oxygen 
and  assimilating  the  carbon  thus  obtained,  the  presence  of 
chlorophyll,  and  the  cell  walls  formed  of  cellulose,  are  char- 
acteristics constant  in  all  typical  plants.  In  addition,  the 
fixed  life  of  plants,  and  their  general  use  of  inorganic  sub- 
stances for  food  instead  of  organic,  are  characteristics 
readily  observed  and  practically  characteristic  of  many- 
celled  plants.  When  the  thousands  of  kinds  of  one-celled 
organisms  are  compared,  however,  it  is  often  a  matter  of 
great  difficulty  or  of  real  impossibility  to  say  whether  a 
given  organism  should  be  assigned  to  the  plant  kingdom 
or  to  the  animal  kingdom.  In  general  the  distinctive 
characters  of  plants  are  grouped  around  the  loss  of  the 
power  of  locomotion  and  related  to  or  dependent  upon  it. 

67.  Living  organic  matter  and  inorganic  matter. — It  would 
seem  to  be  an  easy  matter  to  distinguish  an  organism — that 
is,  a  living  animal  or  plant— from  an  inorganic  substance.  It 
is  easy  to  distinguish  a  dove  or  a  sunflower  from  stone,  and 
practically  there  never  is  any  difficulty  in  making  such  dis- 
tinctions. But  when  we  try  to  define  living  organic  matter, 


THE  PRIMARY  CONDITIONS  OF  ANIMAL  LIFE     H3 

and  to  describe  those  characteristics  which  are  peculiar  to 
it,  which  absolutely  distinguish  it  from  inorganic  matter, 
we  meet  with  some  difficulties.  At  least  many  of  the  char- 
acteristics commonly  ascribed  to  organisms,  as  peculiar  to 
them,  are  not  so.  The  possession  of  organs,  or  the  composi- 
tion of  the  body  of  distinct  parts,  each  with  a  distinct  func- 
tion, but  all  working  together,  and  depending  on  each  other, 
is  as  true  of  a  steam-engine  as  of  a  horse.  That  the  work 
done  by  the  steam-engine  depends  upon  fuel  is  true ;  but 
so  it  is  that  the  work  done  by  the  horse  depends  upon  fuel, 
or  food  as  we  call  it  in  the  case  of  the  animal.  The  oxida- 
tion or  burning  of  this  fuel  in  the  engine  is  wholly  compar- 
able with  the  oxidation  of  the  food,  or  the  muscle  and  fat 
it  is  turned  into,  in  the  horse's  body.  The  composition  of 
the  bodies  of  animals  and  plants  of  tiny  structural  units, 
the  cells,  is  in  many  ways  comparable  with  the  composition 
of  some  rocks  of  tiny  structural  units,  the  crystals.  But 
not  to  carry  such  rather  quibbling  comparisons  too  far,  it 
may  be  said  that  organisms  are  distinguished  from  organic 
substances  by  the  following  characteristics  :  Organization ; 
the  power  to  make  over  inorganic  substances  into  organic 
matter,  or  the  changing  of  organic  matter  of  one  kind,  as 
plant  matter,  into  another  kind,  as  animal  matter ;  motion, 
the  power  of  spontaneous  movement  in  response  to  stimuli ; 
sensation,  the  power  of  being  sensible  of  external  stimuli ; 
reproduction,  the  power  of  producing  new  beings  like  them- 
selves ;  and  adaptation,  the  power  of  responding  to  external 
conditions  in  a  way  useful  to  the  organism.  Through  adap- 
tation organisms  continue  to  exist  despite  the  changing  of 
conditions.  If  the  conditions  surrounding  an  inorganic 
body  change,  even  gradually,  the  inorganic  body  does  not 
change  to  adapt  itself  to  these  conditions,  but  resists  them 
until  no  longer  able  to  do  so,  when  it  loses  its  identity  or 
integrity. 


CHAPTEE  VII 

THE    CROWD    OF   ANIMALS    AND   THE    STRUGGLE    FOR 
EXISTENCE 

68.  The  crowd  of  animals. — All  animals  feed  upon  living 
organisms,  or  on  their  dead  bodies.  Hence  each  animal 
throughout  its  life  is  busy  with  the  destruction  of  other 
organisms,  or  with  their  removal  after  death.  If  those 
creatures  upon  which  others  feed  are  to  hold  their  own,  there 
must  be  enough  born  or  developed  to  make  good  the  drain 
upon  their  numbers.  If  the  plants  did  not  fill  up  their 
ranks  and  make  good  their  losses,  the  animals  that  feed 
on  them  would  perish.  If  the  plant-eating  animals  were 
destroyed,  the  flesh-eating  animals  would  in  turn  disappear. 
But,  fortunately,  there  is  a  vast  excess  in  the  process  of 
reproduction.  More  plants  sprout  than  can  find  room  to 
grow.  More  animals  are  born  than  can  possibly  survive. 
The  process  of  increase  among  animals  is  correctly  spoken 
of  as  multiplication.  Each  species  tends  to  increase  in 
geometric  ratio,  but  as  it  multiplies  its  members  it  finds 
the  world  already  crowded  with  other  species  doing  the 
same  thing.  A  single  pair  of  any  species  whatsoever,  if  not 
restrained  by  adverse  conditions,  would  soon  increase  to 
such  an  extent  as  to  fill  the  whole  world  with  its  progeny. 
An  annual  plant  producing  two  seeds  only  would  have 
1,048,576  descendants  at  the  end  of  twenty-one  years,  if 
each  seed  sprouted  and  matured.  The  ratio  of  increase  is 
therefore  a  matter  of  minor  importance.  It  is  the  ratio  of 
net  increase  above  loss  which  determines  the  fate  of  a  spe- 
cies. Those  species  increase  in  numbers  whose  gain  exceeds 
114 


THE  STRUGGLE  FOR  EXISTENCE  H5 

the  death  rate,  and  those  which  "  live  beyond  their  means  " 
must  sooner  or  later  disappear.  One  of  the  most  abundant 
of  birds  is  the  fulmar  petrel,  which  lays  but  one  egg  yearly. 
It  has  but  few  enemies,  and  this  low  rate  of  increase  suf- 
fices to  cover  the  seas  within  its  range  with  petrels. 

It  is  difficult  to  realize  the  inordinate  numbers  in  which 
each  species  would  exist  were  it  not  for  the  checks  produced 
by  the  presence  of  other  animals.  Certain  Protozoa  at  their 
normal  rate  of  increase,  if  none  were  devoured  or  destroyed, 
might  fill  the  entire  ocean  in  about  a  week.  The  conger- 
eel  lays,  it  is  said,  15,000,000  eggs.  If  each  egg  grew 
up  to  maturity  and  reproduced  itself  in  the  same  way  in 
less  than  ten  years  the  sea  would  be  solidly  full  of  conger- 
eels.  If  the  eggs  of  a  common  house-fly  should  develop,  and 
each  of  its  progeny  should  find  the  food  and  temperature  it 
needed,  with  no  loss  and  no  destruction,  the  people  of  a  city  in 
which  this  might  happen  could  not  get  away  soon  enough  to 
escape  suffocation  from  a  plague  of  flies.  Whenever  any  in- 
sect is  able  to  develop  a  large  percentage  of  the  eggs  laid,  it 
becomes  at  once  a  plague.  Thus  originate  plagues  of  grass- 
hoppers, locusts,  and  caterpillars.  But  the  crowd  of  life  is 
such  that  no  great  danger  exists.  The  scavenger  destroys 
the  decaying  flesh  where  the  fly  would  lay  its  eggs.  Minute 
creatures,  insects,  bacteria,  Protozoa  are  parasitic  within 
the  larva  and  kill  it.  Millions  of  flies  perish  for  want  of 
food.  Millions  more  are  destroyed  by  insectivorous  birds, 
and  millions  are  slain  by  parasites.  The  final  result  is  that 
from  year  to  year  the  number  of  flies  does  not  increase. 
Linnaeus  once  said  that  "  three  flies  would  devour  a  dead 
horse  as  quickly  as  a  lion."  Equally  soon  would  it  be  de- 
voured by  three  bacteria,  for  the  decay  of  the  horse  is  due 
to  the  decomposition  of  its  flesh  by  these  microscopic  plants 
which  feed  upon  it.  "  Even  slow-breeding  man,"  says  Dar- 
win, "  has  doubled  in  twenty-five  years.  At  this  rate  in  less 
than  a  thousand  years  there  would  literally  not  be  standing 
room  for  his  progeny.  The  elephant  is  reckoned  the  slow- 


116  ANIMAL  LIFE 

est  breeder  of  all  known  animals.  It  begins  breeding  when 
thirty  years  old  and  goes  on  breeding  until  ninety  years 
old,  bringing  forth  six  young  in  the  interval,  and  surviving 
till  a  hundred  years  old.  If  this  be  so,  after  about  eight 
hundred  years  there  would  be  19,000,000  elephants  alive, 
descended  from  the  first  pair."  A  few  years  more  of  the 
unchecked  multiplication  of  the  elephant  and  every  foot  of 
land  on  the  earth  would  be  covered  by  them. 

Yet  the  number  of  elephants  does  not  increase.  In  gen- 
eral, the  numbers  of  every  species  of  animal  in  the  state  of 
Nature  remain  about  stationary.  Under  the  influence  of 
man  most  of  them  slowly  diminish.  There  are  about  as 
many  squirrels  in  the  forest  one  year  as  another,  about  as 
many  butterflies  in  the  field,  about  as  many  frogs  in  the 
pond.  Wolves,  bears,  deer,  wild  ducks,  singing  birds,  fishes, 
tend  to  grow  fewer  and  fewer  in  inhabited  regions,  because 
the  losses  from  the  hand  of  man  are  added  to  the  losses  in 
the  state  of  Nature. 

It  has  been  shown  that  at  the  normal  rate  in  increase  of 
English  sparrows,  if  none  were  to  die  save  of  old  age,  it 
would  take  but  twenty  years  to  give  one  sparrow  to  every 
square  inch  in  the  State  of  Indiana.  Such  an  increase  is 
actually  impossible,  for  more  than  a  hundred  other  species 
of  similar  birds  are  disputing  the  same  territory  with  the 
power  of  increase  at  a  similar  rate.  There  can  not  be  food 
and  space  for  all.  With  such  conditions  a  struggle  is  set 
up  between  sparrow  and  sparrow,  between  sparrow  and 
other  birds,  and  between  sparrow  and  the  conditions  of  life. 
Such  a  conflict  is  known  as  the  struggle  for  existence. 

69.  The  struggle  for  existence.— The  struggle  for  exist- 
ence is  threefold:  (a)  among  individuals  of  one  species, 
as  sparrow  and  sparrow ;  ( #)  between  individuals  of  differ- 
ent species,  as  sparrow  with  bluebird  or  robin  ;  and  (c)  with 
the  conditions  of  life,  as  the  effort  of  the  sparrow  to  keep 
warm  in  winter  and  to  find  water  in  summer.  All  three 
forms  of  this  struggle  are  constantly  operative  and  with 


THE  STRUGGLE  FOR  EXISTENCE 

every  species.  In  some  regions  the  one  phase  may  be  more 
destructive,  in  others  another.  Where  the  conditions  of 
life  are  most  easy,  as  in  the  tropics,  the  struggle  of  species 
with  species,  of  individual  with  individual,  is  the  most 
severe. 

No  living  being  can  escape  from  any  of  these  three 
phases  of  the  struggle  for  existence.  For  reasons  which  we 
shall  see  later,  it  is  not  well  that  any  should  escape,  for  "  the 
sheltered  life,"  the  life  withdrawn  from  the  stress  of  effort, 
brings  the  tendency  to  degeneration. 

Because  of  the  destruction  resulting  from  the  struggle 
for  existence,  more  of  every  species  are  born  than  can 
possibly  find  space  or  food  to  mature.  The  majority  fail 
to  reach  their  full  growth  because,  for  one  reason  or  an- 
other, they  can  not  do  so.  All  live  who  can.  Each  strives 
to  feed  itself,  to  save  its  own  life,  to  protect  its  young. 
But  with  all  their  efforts  only  a  portion  of  each  species 
succeed. 

70.  Selection  by  Nature. — But  the  destruction  in  Nature 
is  not  indiscriminate.  In  the  long  run  those  least  fitted  to 
resist  attack  are  the  first  to  perish.  It  is  the  slowest  ani- 
mal which  is  soonest  overtaken  by  those  which  feed  upon 
it.  It  is  the  weakest  which  is  crowded  away  from  the  feed- 
ing-place by  its  associates.  It  is  the  least  adapted  which  is 
first  destroyed  by  extremes  of  heat  and  cold.  Just  as  a 
farmer  improves  his  herd  of  cattle  by  destroying  his  weak- 
est or  roughest  calves,  reserving  the  strong  and  fit  for  par- 
entage, so,  on  an  inconceivably  large  scale,  the  forces  of 
Nature  are  at  work  purifying,  strengthening,  and  fitting  to 
their  surroundings  the  various  species  of  animals.  This 
process  has  been  called  natural  selection,  or  the  survival  of 
the  fittest.  But  by  fittest  in  this  sense  we  mean  only  best 
adapted  to  the  surroundings,  for  this  process,  like  others  in 
Nature,  has  itself  no  necessarily  moral  element.  The  song- 
bird becomes  through  this  process  more  fit  for  the  song-bird 
life,  the  hawk  becomes  more  capable  of  killing  and  tear- 
9 


ANIMAL  LIFE 

ing,  and  the  woodpecker  better  fitted  to  extract  grubs  from 
the  tree. 

In  the  struggle  of  species  with  species  one  may  gain  a 
little  one  year  and  another  the  next,  the  numbers  of  each 
species  fluctuating  a  little  with  varying  circumstances,  but 
after  a  time,  unless  disturbed  by  the  hand  of  man,  a  point 
will  be  reached  when  the  loss  will  almost  exactly  balance 
the  increase.  This  produces  a  condition  of  apparent  equi- 
librium. The  equilibrium  is  broken  when  any  individual  or 
group  of  individuals  becomes  capable  of  doing  something 
more  than  hold  its  own  in  the  struggle  for  existence. 

When  the  conditions  of  life  become  adverse  to  the  exist- 
ence of  a  species  it  has  three  alternatives,  or,  better,  one  of 
three  things  happens,  namely,  migration,  adaptation,  extinc- 
tion. The  migration  of  birds  and  some  other  animals  is  a 
systematic  changing  of  environment  when  conditions  are 
unfavorable  to  life.  When  the  snow  and  ice  come,  the  fur- 
seal  forsakes  the  islands  on  which  it  breeds,  and  which  are 
its  real  home,  and  spends  the  rest  of  the  year  in  the  open 
sea,  returning  at  the  close  of  winter.  Some  other  animals 
migrate  irregularly,  removing  from  place  to  place  as  condi- 
tions become  severe  or  undesirable.  The  Eocky  Mountain 
locusts,  which  breed  on  the  great  plateau  along  the  eastern 
base  of  the  Rocky  Mountains,  sometimes  increase  so  rapidly 
in  numbers  that  they  can  not  find  enough  food  in  the  scanty 
vegetation  of  this  region.  Then  great  hosts  of  them  fly 
high  into  the  air  until  they  meet  an  air  current  moving 
toward  the  southeast.  The  locusts  are  borne  by  this  cur- 
rent or  wind  hundreds  of  miles,  until,  when  they  come  to 
the  great  grain-growing  Mississippi  Valley,  they  descend 
and  feed  to  their  hearts'  content,  and  to  the  dismay  of  the 
Nebraska  and  Kansas  farmer.  These  great  forced  migra- 
tions used  to  occur  only  too  often,  but  none  has  taken  place 
since  1878,  and  it  is  probable  that  none  will  ever  occur 
again.  With  the  settlement  of  the  Rocky  Mountain  plateau 
by  farmers,  food  is  plenty  at  home.  And  the  constant  fight- 


THE  STRUGGLE  FOR  EXISTENCE  H9 

ing  of  the  locusts  by  the  farmers,  by  plowing  up  their  eggs, 
and  crushing  and  burning  the  young  hoppers,  keeps  down 
their  numbers. 

Another  animal  of  interesting  migratory  habits  is  the 
lemming,  a  mouse-like  animal  nearly  as  large  as  a  rat,  which 
lives  in  the  arctic  regions.  At  intervals  varying  from  five 
to  twenty  years  the  cultivated  lands  of  Norway  and  Sweden, 
where  the  lemming  is  ordinarily  unknown,  are  overrun  by 
vast  numbers  of  these  little  animals.  They  come  as  an 
army,  steadily  and  slowly  advancing,  always  in  the  same 
direction,  and  "  regardless  of  all  obstacles,  swimming  across 
streams  and  even  lakes  of  several  miles  in  breadth,  and 
committing  considerable  devastation  on  their  line  of  march 
by  the  quantity  of  food  they  consume.  In  their  turn  they 
are  pursued  and  harassed  by  crowds  of  beasts  and  birds  of 
prey,  as  bears,  wolves,  foxes,  dogs,  wild  cats,  stoats,  weasels, 
eagles,  hawks,  and  owls,  and  never  spared  by  man ;  even 
the  domestic  animals  not  usually  predaceous,  as  cattle, 
foals,  and  reindeer,  are  said  to  join  in  the  destruction, 
stamping  them  to  the  ground  with  their  feet  and  even  eat- 
ing their  bodies.  Numbers  also  die  from  disease  apparently 
produced  from  overcrowding.  None  ever  return  by  the 
course  by  which  they  came,  and  the  onward  march  of  the 
survivors  never  ceases  until  they  reach  the  sea,  into  which 
they  plunge,  and  swimming  onward  in  the  same  direction 
as  before  perish  in  the  waves."  One  of  these  great  migra- 
tions lasts  for  from  one  to  three  years.  But  it  always  ends 
in  the  total  destruction  of  the  migrating  army.  But  the 
migration  may  be  of  advantage  to  the  lemmings  which  re- 
main in  the  original  breeding  grounds,  leaving  them  with 
enough  food,  so  that,  on  the  whole,  the  migration^ results  in 
gain  to  the  species. 

But  most  animals  can  not  migrate  to  their  betterment. 
In  that  case  the  only  alternatives  are  adaptation  or  destruc- 
tion. Some  individuals  by  the  possession  of  slight  advan- 
tageous variations  of  structure  are  able  to  meet  the  new 


120  ANIMAL  LIFE 

demands  and  survive,  the  rest  die.  The  survivors  produce 
young  similarly  advantageously  different  from  the  general 
type,  and  the  adaptation  increases  with  successive  genera- 
tions. 

71.  Adjustment  to  surroundings  a  result  of  natural  selec- 
tion.— To  such  causes  as  these  we  must  ascribe  the  nice 
adjustment  of  each  species  to  its  surroundings.     If  a  species 
or  a  group  of  individuals  can  not  adapt  itself  to  its  environ- 
ment, it  will  be  crowded  out  by  others  that  can  do  so.     The 
former  will  disappear  entirely  from  the  earth,  or  else  will  be 
limited  to  surroundings  with  which  it  comes  into  perfect 
adjustment.     A  partial  adjustment  must  with  time  become 
a  complete  one,  for  the  individuals  not  adapted  will  be 
exterminated  in  the  struggle  for  life.     In  this  regard  very 
small  variations  may  lead  to  great  results.     A  side  issue 
apparently  of  little  consequence  may  determine  the  fate  of 
a  species.     Any  advantage,  no  matter  how  small,  will  turn 
the  scale  of  life  in  favor  of  its  possessor  and  his  progeny. 
"  Battle  within  battle,"  says  a  famous  naturalist,  "  must  be 
continually  recurring,  with  varying  success.     Yet  in  the 
long  run  the  forces  are  so  nicely  balanced  that  the  face  of 
Nature  remains  for  a  long  time  uniform,  though  assuredly 
the  merest  trifle  would  give  the  victory  to  one  organic  being 
over  another." 

72.  Artificial  selection.— It  has  been  long  known  that  the 
nature  of  a  herd  or  race  of  animals  can  be  materially  altered 
by  a  conscious  selection  on  the  part  of  man  of  these  indi- 
viduals which  are  to  become  parents.     To  "  weed  out "  a 
herd  artificially  is  to  improve  its  blood.     To  select  for  re- 
production the  swiftest  horses,  the  best  milk  cows,  the  most 
intelligent  dogs,  is  to  raise  the  standard  of  the  herd  or 
race  in  each  of  these  respects  by  the  simple  action  of  hered- 
ity.    Artificial  selection  has  been  called  the  "magician's 
wand,"  by  which  the  breeder  can  summon  up  whatever 
animal  form  he  will.     If  the  parentage  is  chosen  to  a  defi- 
nite end,  the  process  of  heredity  will  develop  the  form 


THE  STRUGGLE  FOR  EXISTENCE 

desired  by  a  force  as  unchanging  as  that  by  which  a  stream 
turns  a  mill. 

From  the  wild  animals  about  him  man  has  developed 
the  domestic  animals  which  he  finds  useful.  The  dog 
which  man  trains  to  care  for  his  sheep  is  developed  by 
selection  from  the  most  tractable  progeny  of  the  wolf  which 
once  devoured  his  flocks.  By  the  process  of  artificial  selec- 
tion those  individuals  that  are  not  useful  to  man  or  pleas- 
ing to  his  fancy  have  been  destroyed,  and  those  which  con- 
tribute to  his  pleasure  or  welfare  have  been  preserved  and 
allowed  to  reproduce  their  kind.  The  various  fancy  breeds 
of  pigeons — the  carriers,  pouters,  tumblers,  ruff-necks,  and 
fan-tails — are  all  the  descendants  of  the  wild  dove  of  Eu- 
rope (Columba  livid].  These  breeds  or  races  or  varieties 
have  been  produced  by  artificial  selection.  So  it  is  with 
the  various  breeds  of  cattle  and  of  hogs  and  of  horses 
and  dogs. 

In  this  artificial  selection  new  variations  are  more  rap- 
idly produced  than  in  Nature  by  means  of  intercrossing 
different  races,  and  by  a  more  rapid  weeding  out  of  un- 
favorable— that  is,  of  undesirable — variations.  The  rapid 
production  of  variations  and  the  careful  preservation  of 
the  desirable  ones  and  rigid  destruction  of  undesirable 
ones  are  the  means  by  which  many  races  of  domestic  ani- 
mals are  produced.  This  is  artificial  selection. 

73.  Dependence  of  species  on  species.— There  was  intro- 
duced into  California  from  Australia,  on  young  orange  trees, 
a  few  years  ago,  an  insect  pest  called  the  cottony  cushion 
scale  (Iccrya  purchasi).  This  pest  increased  in  numbers 
with  extraordinary  rapidity,  and  in  four  or  five  years  threat- 
ened to  destroy  completely  the  great  orange  orchards  of 
California.  Artificial  remedies  were  of  little  avail.  Finally, 
an  entomologist  was  sent  to  Australia  to  find  out  if  this 
scale  insect  had  not  some  special  natural  enemy  in  its 
native  country.  It  was  found  that  in  Australia  a  certain 
species  of  lady-bird  beetle  attacked  and  fed  on  the  cottony 


122  ANIMAL  LIFE 

cushion  scales  and  kept  them  in  check.  Some  of  these 
lady-birds  ( Vedalia  cardinalis)  were  brought  to  California 
and  released  in  a  3cale-infested  orchard.  The  lady-birds, 
having  plenty  of  food,  thrived  and  produced  many  young. 
Soon  the  lady-birds  were  in  such  numbers  that  numbers  of 
them  could  be  distributed  to  other  orchards.  In  two  or 
three  years  the  Vedalias  had  become  so  numerous  and 
widely  distributed  that  the  cottony  cushion  scales  began  to 
dimmish  perceptibly,  and  soon  the  pest  was  nearly  wiped 
out.  But  with  the  disappearance  of  the  scales  came  also  a 
disappearance  of  the  lady-birds,  and  it  was  then  discovered 
that  the  Vedalias  fed  only  on  cottony  cushion  scales  and 
could  not  live  where  the  scales  were  not.  So  now,  in  order 
to  have  a  stock  of  Vedalias  on  hand  in  California  it  is  neces- 
sary to  keep  protected  some  colonies  of  the  cottony  cushion 
scale  to  serve  as  food.  Of  course,  with  the  disappearance 
of  the  predaceous  lady-birds  the  scale  began  to  increase 
again  in  various  parts  of  the  State,  but  with  the  sending  of 
Vedalias  to  these  localities  the  scale  was  again  crushed. 
How  close  is  the  interdependence  of  these  two  species ! 

Similar  relations  can  be  traced  in  every  group  of  ani- 
mals. When  the  salmon  cease  to  run  in  the  Sacramento 
Eiver  in  California  the  otter  which  feeds  on  them  takes,  it 
is  said,  to  robbing  the  poultry-yards ;  and  the  bear,  which 
also  feeds  on  fish,  strikes  out  for  other  game,  taking  fruit 
or  chickens  or  bee-hives,  whatever  he  may  find. 


CHAPTEK  VIII 

ADAPTATIONS 

74.  Origin  of  adaptations. — The  strife  for  place  in  the 
crowd  of  animals  makes  it  necessary  for  each  one  to  adjust 
itself  to  the  place  it  holds.     As  the  individual  becomes 
fitted  to  its  condition,  so  must  the  species  as  a  whole.     The 
species  is  therefore  made  up  of  individuals  that  are  fitted 
or  may  become  fitted  for  the  conditions  of  life.     As  the 
stress  of  existence  becomes  more  severe,  the  individuals  fit 
to  continue  the   species  are   chosen  more   closely.     This 
choice  is  the  automatic  work  of  the  conditions  of  life,  but 
it  is  none  the  less  effective  in  its  operations,  and  in  the 
course  of  centuries  it  becomes  unerring.     When  conditions 
change,  the  perfection  of  adaptation  in  a  species  may  be 
the  cause  of  its  extinction.     If  the  need  of  a  special  fitness 
can  not  be  met  immediately,  the  species  will  disappear. 
For  example,  the  native  sheep  of  England  have  developed 
a  long  wool  fitted  to  protect  them  in  a  cool,  damp  climate. 
Such  sheep  transferred  to  Cuba  died  in  a  short  time,  leav- 
ing no  descendants.     The  warm  fleece,  so  useful  in  Eng- 
land, rendered  them  wholly  unfit  for  survival  in  the  tropics. 
It  is  one  advantage  of  man,  as  compared  with  other  forms 
of  life,  that  so  many  of  his  adaptations  are  external  to  his 
structure,  and  can  be  cast  aside  when  necessity  arises. 

75.  Classification  of  adaptations.— The  various  forms   of 
adaptations  may  be  roughly  divided  into  five  classes,  as  fol- 
lows :  (a)  food  securing,  (#)  self-protection,  (c)  rivalry,  (d) 
defense  of  young,  (e)  surroundings. 

The  few  examples  which  are  given  under  each  class, 

123 


124  ANIMAL  LIFE 

some  of  them  striking,  some  not  especially  so,  are  mostly 
chosen  from  the  vertebrates  and  from  the  insects,  because 
these  two  groups  of  animals  are  the  groups  with  which  be- 
ginning students  of  zoology  are  likely  to  be  familiar,  and 
the  adaptations  referred  to  are  therefore  most  likely  to  be 
best  appreciated.  Quite  as  good  and  obvious  examples  could 
be  selected  from  any  other  groups  of  animals.  The  student 


PIG.  54.— The  deep-sea  angler  (Corynolophus  reinhardti),  which  has  a  dorsal  spine 
modified  to  be  a  luminous  "fishing-rod  and  lure,"  attracting  lantern-fishes 
(Echiostoma  and  ^Et?wphora).  An  extraordinary  adaptation  for  securing  food. 
(The  angler  is  drawn  after  a  figure  of  L^TKEN'S.) 

will  find  good  practice  in  trying  to  discover  examples  shown 
by  the  animals  with  which  he  may  be  familiar.  That  all 
or  any  part  of  the  body  structure  of  any  animal  can  be 
called  with  truth  an  example  of  adaptation  is  plain  from 
what  we  know  of  how  the  various  organs  of  the  animal 
body  have  come  to  exist.  But  by  giving  special  attention 
to  such  adaptations  as  are  plainly  obvious,  beginning  stu- 


ADAPTATIONS 


125 


dents  may  be  put  in  the 
way  of  independent  ob- 
servation along  an  ex- 
tremely interesting  and 
attractive  line  of  zoolog- 
ical study. 

76.  Adaptations  for 
securing  food.  —  For  the 
purpose  of  capture  of 
their  prey,  some  carniv- 
orous animals  are  pro- 
vided with  strong  claws, 
sharp  teeth,  hooked 
beaks,  and  other  struc- 
tures familiar  to  us  in 
the  lion,  tiger,  dog,  cat, 
owl,  and  eagle.  Insect- 
eating  mammals  have 
contrivances  especially 


FIG.  55.— The  brown  pelican,  showing  gular 
sac,  which  it  uses  in  catching  and  holding 
fishes  that  form  its  food. 


Fie.  56.— Foot  of  the  bald  eagle,  show- 
ing claws  for  seizing  its  prey, 
(CHAPMAN.) 


adapted  for  the  catching  of  insects.  The 
ant-eater,  for  example,  has  a 
curious,  long  sticky  tongue 
which  it  thrusts  forth  from 
its  cylindrical  snout  deep 
into  the  recesses  of  the  ant- 
hill, bringing  it  out  with  its 
sticky  surface  covered  with 
ants.  Animals  which  feed  on 
nuts  are  fitted  with  strong 
teeth  or  beaks  for  crack- 
ing them.  Similar  teeth  are 
found  in  those  fishes  which 
feed  on  crabs,  snails,  or  sea-ur- 
chins. Those  mammals  like 
the  horse  and  cow,  that 
feed  on  plants,  have  usually 


FIG.  57.— Giraffes  feeding. 


ADAPTATIONS 


127 


broad  chisel-like  incisor  teeth  for  cutting  off  the  foliage, 
and  teeth  of  very  similar  form  are  developed  in  the  dif- 
ferent   groups   of  plant- 
eating    fishes.       Molar 
teeth  are  found  when  it 


FIG.  58. — Scorpion,  showing  the  special  devel- 
opment of  certain  mouth  parts  (the  maxil- 
lary palpi)  as  pincer-like  organs  for  grasp- 
ing prey.  At  the  posterior  tip  of  the  body 
is  the  poisonous  sting. 


PIG.  59.— Head  of  mosquito  (fe- 
male), showing  the  piercing 
needle-like  month  parts  which 
compose  the  "bill." 


is  necessary  that  the  food  should  be  crushed  or  chewed, 

and  the   sharp   canine  teeth  go   with  a  flesh  diet.     The 

long   neck   of    the    giraffe 

(Fig.  57)   enables  it  to 

browse   on    the    foliage   of 

trees. 

Insects  like  the  leaf- 
beetles  and  the  grasshop- 
pers, that  feed  on  the 
foliage  of  plants,  have  a 

.  .       .  ,  FIG.  60.— The  praying-horse  (Mantis)  with 

pair     Of      jaWS,      broad      but         fore  legs  developed  as  grasping  organs. 


128 


ANIMAL  LIFE 


sharply  edged,  for  cutting  off  bits  of  leaves  and  stems. 
Those  which  take  only  liquid  food,  as  the  butterflies  and 
sucking-bugs,  have  their  mouth  parts  modified  to  form  a 
slender,  hollow  sucking  beak  or  proboscis,  which  can  be 

thrust  into  a  flower  nectary, 
or  into  the  green  tissue  of 
plants  or  the  flesh  of  animals, 
to  suck  up  nectar  or  plant  sap 
or  blood,  depending  on  the 
special  food  habits  of  the  in- 
sect. The  honey-bee  has  a 
very  complicated  equipment 
of  mouth  parts  fitted  for  tak- 
ing either  solid  food  like  pol- 
len, or  liquid  food  like  the 
nectar  of  flowers.  The  mos- 
quito has  a  "bill"  (Fig.  59) 
composed  of  six  sharp,  slender 
needles  for  piercing  and  lac- 
erating the  flesh,  and  a  long 
tubular  under  lip  through 
which  the  blood  can  flow  into 
the  mouth.  Some  predaceous 
insects,  as  the  praying-horse 
(Fig.  60),  have  their  fore  legs 
developed  into  formidable 
grasping  organs  for  seizing  and 
holding  their  prey. 

77.  Adaptation    for     self-de- 
fense.— For  self-protection,  car- 

Fio.  61.— Acorns  put  into  bark  of  tree         .  .          * 

by    the    Californian    woodpecker     niVOrOUS  animals  US6    the   Same 

(Meianerpes  formicivorus  bairdii).    weapons  to  defend  themselves 

—From  photograph,  Stanford  Uni-  . r 

versity,  California,  which    serve    to    secure    their 

prey;    but    these    as    well  as 

other  animals   may  protect  themselves  in  other  fashions. 
Most  of  the  hoofed  animals  are  provided  with  horns,  struc- 


ADAPTATIONS 


129 


FIG.  62.— Section  of  bark  of  live  oak  tree  with  acorns  placed  in  it  by  the  Californian 
woodpecker  (Melanerpes  formicivorus  bairdii). — From  photograph,  Stanford 
University,  California. 

tures  useless  in  procuring  food  but  often  of  great  effective- 
ness as  weapons  of  defense.  To  the  category  of  structures 
useful  for  self-defense  belong  the  many  peculiarities  of  col- 
oration known  as  "recognition  marks."  These  are  marks, 
10 


130 


ANIMAL  LIFE 


not  otherwise  useful,  which,  are  supposed  to  enable  mem- 
bers of  any  one  species  to  recognize  their  own  kind  among 
the  mass  of  animal  life.  To  this  category  belongs  the 
black  tip  of  the  weasel's  tail,  which  re- 
mains the  same  whatever  the  changes 
in  the  outer  fur.  Another  example  is 
seen  in  the  white  outer  feathers  of  the 
tail  of  the  meadow-lark  as  well  as  in 
certain  sparrows  and  warblers.  The 
white  on  the  skunk's  back  and  tail 
serves  the  same  purpose  and  also  as  a 
warning.  It  is  to  the  skunk's  advan- 
tage not  to  be  hidden,  for  to  be  seen  in 
the  crowd  of  animals  is  to  be  avoided 
by  them.  The  songs  of  birds  and  the 
calls  of  various  creatures  serve  also  as 
recognition  marks.  Each  species  knows 
and  heeds  its  own  characteristic  song 
or  cry,  and  it  is  a  source  of  mutual 
protection.  The  fur-seal  pup  knows 
its  mother's  call,  even  though  ten  thou- 
sand other  mothers  are  calling  on  the 
rookery. 

The  ways  in  which  animals  make 
themselves  disagreeable  or  dangerous 
to  their  captors  are  almost  as  varied  as  the  animals  them- 
selves. Besides  the  teeth,  claws,  and  horns  of  ordinary 
attack  and  defense,  we  find  among  the  mammals  many 
special  structures  or  contrivances  which  serve  for  de- 
fense through  making  their  possession  unpleasant.  The 
scent  glands  of  the  skunk  and  its  relatives  are  noticed 
above.  The  porcupine  has  the  bristles  in  its  fur  specialized 
as  quills,  barbed  and  detachable.  These  quills  fill  the 
mouth  of  an  attacking  fox  or  wolf,  and  serve  well  the  pur- 
pose of  defense.  The  hedgehog  of  Europe,  an  animal  of 
different  nature,  being  related  rather  to  the  mole  than  to 


FIG.  63.-Centiped.  The 
foremost  pair  of  legs  is 
modified  to  be  a  pair  of 
seizing  and  stinging  or- 
gans. An  adaptation 
for  self-defense  and  for 
securing  food. 


ADAPTATIONS 


131 


the  squirrel,  has  a  similar  armature  of  quills.     The  armadillo 
of  the  tropics  has  movable  shields,  and  when  it  withdraws  its 


FIG.  64.— Flying  fishes.  (The  upper  one  a  species  of  Cypselurus,  the  lower  of  Exoca- 
tus.}  These  fishes  escape  from  their  enemies  by  leaping  into  the  air  and  sailing 
or  "flying"  long  distances. 

head  (which  is  also  defended  by  a  bony  shield)  it  is  as  well 
protected  as  a  turtle. 


FIG.  65.— The  horned  toad  (Phrynosoma  blammllei).    The  spiny  covering  repels  many 

enemies. 

Special  organs  for  defense  of  this  nature  are  rare  among 
birds,  but  numerous  among   reptiles.     The  turtles  are  all 


132  ANIMAL  LIFE 

protected  by  bony  shields,  and  some  of  them,  the  box-tur- 
tles, may  close  their  shields  almost  hermetically.  The 
snakes  broaden  their  heads,  swell  their  necks,  or  show  their 
forked  tongues  to  frighten  their  enemies.  Some  of  them 


FIG.  66.— Nokee  or  poisonous  scorpion-fish  (Emmydrichthys  vulcanus)  with  poison- 
ous spines,  from  Tahiti. 

are  further  armed  with  fangs  connected  with  a  venom  gland, 
so  that  to  most  animals  their  bite  is  deadly.  Besides  its 
fangs  the  rattlesnake  has  a  rattle  on  the  tail  made  up  of  a 


FIG.  67. — Mad  torn  (Schilbeodes  furiosus)  with  poisoned  pectoral  spine. 

succession  of  bony  clappers,  modified  vertebrae,  and  scales, 
by  which  intruders  are  warned  of  their  presence.  This 
sharp  and  insistent  buzz  is  a  warning  to  animals  of  other 
species  and  a  recognition  signal  to  those  of  its  own  kind. 


ADAPTATIONS  133 

Even  the  fishes  have  many  modes  of  self-defense  through 
giving  pain  or  injury  to  those  who  would  swallow  them. 
The  cat-fishes  or  horned  pouts  when  attacked  set  immov- 
ably the  sharp  spine  of  the 
pectoral  fin,  indicting  a 
jagged  wound.  Pelicans 
who  have  swallowed  a  cat- 
fish have  been  known  to 
die  of  the  wounds  inflicted 
by  the  fish's  spine.  In 
the  group  of  scorpion- 
fishes  and  toad-fishes  are 
certain  genera  in  which 
these  spines  are  provided 
with  poison  glands.  These 
may  inflict  very  severe 
wounds  to  other  fishes,  or 
even  to  birds  or  man.  One  of  this  group 
of  poison-fishes  is  the  nokee  (Emmydrich- 
thys.  Fig.  66).  A  group  of  small  fresh- 
water cat-fishes,  known  as  the  mad  toms 
(Fig.  67),  have  also  a  poison  gland  attached 
to  the  pectoral  spine,  and  its  sting  is  most 
exasperating,  like  the  sting  of  a  wasp. 
The  sting-rays  (Fig.  68)  of  many  species  FIG.,  ea— A  sting  ray 
have  a  strong,  jagged  spine  on  the  tail,  %£%££** 
covered  with  slime,  and  armed  with  broad 
saw -like  teeth.  This  inflicts  a  dangerous  wound,  not 
through  the  presence  of  specific  venom,  but  from  the  dan- 
ger of  blood  poisoning  arising  from  the  slime,  and  the 
ragged  or  unclean  cut. 

Many  fishes  are  defended  by  a  coat  of  mail  or  a  coat  of 
sharp  thorns.  The  globe-fishes  and  porcupine-fishes  (Fig. 
69)  are  for  the  most  part  defended  by  spines,  but  their 
instinct  to  swallow  air  gives  them  an  additional  safeguard. 
When  one  of  these  fishes  is  disturbed  it  rises  to  the  surface, 
10 


134 


ANIMAL  LIFE 


gulps  air  until  its  capacious  stomach  is  filled,  and  then 
floats  belly  upward  on  the  surface.  It  is  thus  protected 
from  other  fishes,  though  easily  taken  by  man.  The  torpe- 
do, electric  eel,  electric  cat-fish,  and  star-gazer,  surprise  and 

stagger  their  captors  by 
means  of  electric  shocks. 
In  the  torpedo  or  electric 
ray  (Fig.  70),  found  on 
the  sandy  shores  of  all 
warm  seas,  on  either  side 
of  the  head  is  a  large 
honeycomb-like  structure 
which  yields  a  strong 
electric  shock  whenever 
the  live  fish  is  touched. 
This  shock  is  felt  severe- 
ly if  the  fish  be  stabbed 
with  a  knife  or  metallic 
spear.  The  electric  eel 
of  the  rivers  of  Para- 
guay and  southern  Bra- 
zil is  said  to  give  severe 
shocks  to  herds  of  wild 
horses  driven  through 
the  streams,  and  similar 
accounts  are  given  of  the 
electric  cat-fish  of  the 
Nile. 

FIG.  69. — Porcupine-fish  (Diodon  hystrix),  the  .  , , 

lower  ones  swimming  normally,  the  upper  Among     the      insects, 

one  floating   belly  upward,  with   inflated  the    possession    of    stingS 

Btomach. — Drawn  from  specimens  from  the  •  m-i 

Florida  Keys.  ls  not  uncommon.     The 

wasps  and  bees  are  fa- 
miliar examples  of  stinging  insects,  but  many  other  kinds, 
less  familiar,  are  similarly  protected.  All  insects  have 
their  bodies  covered  with  a  coat  of  armor,  composed  of  a 
horny  substance  called  chitin.  In  some  cases  this  chitin- 


ADAPTATIONS 


135 


ous  coat  is  very  thick  and  serves  to  protect  them  effectu- 
ally. This  is  especially  true  of  the  beetles.  Some  insects 
are  inedible  (as  mentioned  in  Chapter  XII),  and  are  con- 
spicuously colored  so  as  to  be  readily  recognized  by  in- 
sectivorous bird^.  The  birds,  knowing  by  experience  that 
these  insects  are  ill-tasting,  avoid  them.  Others  are  ef- 
fectively concealed  from  their  enemies  by  their  close 
resemblance  in  color  and  marking  to  their  surroundings. 
These  protective  resem- 
blances are  discussed  in 
Chapter  XII. 

78.  Adaptation  for  rivalry. 
— In  questions  of  attack  and 
defense,  the  need  of  meeting 
animals  of  their  own  kind  as 
well  as  animals  of  other 
races  must  be  considered.  In 
struggles  of  species  with 
those  of  their  own  kind,  the 
term  rivalry  may  be  applied. 
Actual  warfare  is  confined 
mainly  to  males  in  the  breed- 
ing season  and  to  polyga- 
mous animals.  Among  those 
in  which  the  male  mates 
with  many  females,  he  must 
struggle  with  other  males  for 
their  possession.  In  all  the 
groups  of  vertebrates  the 
sexes  are  about  equal  in  num- 
bers. Where  mating  exists, 
either  for  the  season  or  for 

life,  this  condition  does  not   involve   serious   struggle   or 
destructive  rivalry. 

Among  monogamous  birds,  or  those  which  pair,  the 
male  courts  the  female  of  his  choice  by  song  and  by  display 


FIG.  70.— Torpedo  or  electric  ray  (Nar- 
cine  brasiliensis),  showing  electric 
cells. 


ADAPTATIONS 


13Y 


of  his  bright  feathers.  The  female  consents  to  be  chosen 
by  the  one  which  pleases  her.  It  is  believed  that  the  hand- 
somest, most  vivacious,  and  most  musical  males  are  the 
ones  most  successful  in  such  courtship.  With  polygamous 
animals  there  i?  intense  rivalry  among  the  males  in  the 
mating  season,  which  in  almost  all  species  is  in  the  spring. 
The  strongest  males  survive  and  reproduce  their  strength. 
The  most  notable  adaptation  is  seen  in  the  superior  size 
of  teeth,  horns,  mane,  or  spurs.  Among  the  polygamous 
fur  seals  (Fig.  71)  and  sea  lions  the  male  is  about  four  times 


FIG.  72. — A  wild  duck  (Aythya)  family.    Male,  female,  and  praecocial  young. 

the  size  of  the  female.  In  the  polygamous  family  of  deer, 
buffalo,  and  the  domestic  cattle  and  sheep,  the  male  is  larger 
and  more  powerfully  armed  than  the  female.  In  the  polyg- 
amous group  to  which  the  hen,  turkey,  and  peacock  belong 
the  males  possess  the  display  of  plumage,  and  the  structures 
adapted  for  fighting,  with  the  will  to  use  them. 

79.  Adaptations  for  the  defense  of  the  young. — The  pro- 
tection of  the  young  is  the  source  of  many  adaptive  struc- 
tures as  well  as  of  the  instincts  by  which  such  structures  are 


138 


ANIMAL  LIFE 


utilized.     In  general,  those  animals  are  highest  in  develop- 
ment, with  best  means  of  holding  their  own  in  the  struggle 


FIG.  73.— The  altricial  nestlings  of  the  Blue  jay  (Cyanocitta  cristata). 


for  life,  that  take  best  care  of  their  young.     The  homes 
of  animals  are  elsewhere  specially  discussed  (see  Chapter 


ADAPTATIONS 


139 


XV),  but  those  instincts  which  lead  to  home-building 
may  all  be  regarded  as  useful  adaptations  in  preserving  the 
young.  Among  the  lower  or  more  coarsely  organized 


FIG.  74. — kangaroo  (Macropiix  mtfns)  with  young  in  pouch. 


140 


ANIMAL  LIFE 


birds,  such  as  the  chicken,  the  duck,  and  the  auk,  as  with 
the  reptiles,  the  young  animal  is  hatched  with  well-devel- 
oped muscular  system  and  sense 
organs,  and  is  capable  of  running 
about,  and,  to  some  extent,  of  feed- 
ing itself.  Birds  of  this  type  are 
known  as  prcecocial  (Fig.  72),  while 
the  name  altricial  (Fig.  73)  is  ap- 
plied to  the  more  highly  organized 
forms,  such  as  the  thrushes,  doves, 
and  song-birds  generally.  With 
these  the  young  are  hatched  in  a 
wholly  helpless  condition,  with  in- 
effective muscles,  deficient  senses, 
and  dependent  wholly  upon  the 
parent.  The  altricial  condition  de- 
mands the  building  of  a  nest,  the 
establishment  of  a  home,  and  the 

FIG.  75. — Egg-case  of  California  , .  -•  »  i     ,1         • 

barn-door  skate  (RajaUnocu-  continued  care  of  one  or  both  of 

lata)  cut  open  to  show  young    the  parents. 

The  very  lowest  mammals  known, 
the  duck-bills  (Monotremes)  of 
Australia,  lay  large  eggs  in  a  strong  shell  like  those  of  a 
turtle,  and  guard  them  with  great  jealousy.  But  with 
almost  all  mammals  the  egg  is  very  small  and  without 
much  food-yolk.  The  egg  begins  its  development  within 
the  body.  It  is  nourished  by  the 
blood  of  the  mother,  and  after  birth 
the  young  is  cherished  by  her,  and 
fed  by  milk  secreted  by  specialized 
glands  of  the  skin.  All  these  features 
are  adaptations  tending  toward  the 
preservation  of  the  young.  In  the 

division  of  mammals  next  lowest  to  the  Monotremes — the 
kangaroo,  opossum,  etc. — the  young  are  born  in  a  very  im- 
mature state  and  are  at  once  seized  by  the  mother  and 


inside.     (Young  issues  natu- 
rally at  one  end  of  the  case.) 


FIG.  76.— Egg-case  of  the  cock- 
roach. 


ADAPTATIONS 


141 


thrust  into  a  pouch  or  fold  of  skin  along  the  abdomen, 
where  they  are  kept  until  they  are  able  to  take  care  of 
themselves  (Fig.  74).  This  is  an  interesting  and  ingenious 
adaptation,  but  less  specialized  and 
less  perfect  an  adaptation  than  the 
conditions  found  in  ordinary  mam- 
mals. 

Among  the  insects,  the  special 
provisions  for  the  protection  and 
care  of  the  eggs  and  the  young  are 
wide-spread  and  various.  Some  of 
those  adaptations  which  take  the 
special  form  of  nests  or  "homes" 
will  be  described  in  a  later  chapter 
(see  Chapter  XV).  The  eggs  of 
the  common  cockroach  are  laid  in 
small  packets  inclosed  in  a  firm  wall 

(Fig.  76).  The  eggs  of  the  great  water-bugs  are  carried  on 
the  back  of  the  male  (Fig.  77) ;  and  the  spiders  lay  their 
eggs  in  a  silken  sac  or  cocoon,  and  some  of  the  ground  or 


FIG.  77.— Giant  water-bug  (Ser- 
phus).  Male  carrying  egga 
on  its  back. 


FIG.  78. — Cocoon  inclosing  the  pupa  of  the  great  Ceanothus  moth.    Spun  of  eilk  by  the 
larva  before  pupation. 

running  spiders  (Lycosida?)  drag  this  egg-sac,  attached  to 
the  tip  of  the  abdomen,  about  with  them.  The  young 
spiders  when  hatched  live  for  some  days  inside  this  sac, 
feeding  on  each  other!  Many  insects  have  long,  sharp, 


142 


ANIMAL  LIFE 


piercing  ovipositors,  by  means  of  which  the  eggs  are  de- 
posited  in  the  ground  or  in  the  leaves  or  stems  of  green 
plants,  or  even  in  the  hard  wood  of  tree-trunks.  Some  of 


the  scale  insects  se- 
crete wax  from  their 
bodies  and  form  a 
large,  of  ten>beautif  ul 
egg-case,  attached  to 
and  nearly  covering  the  body  in 
which  eggs  are  deposited  (Fig. 
79).  The  various  gall  insects  lay 
their  eggs  in  the  soft  tissue  of 
plants,  and  on  the  hatching  of 
the  larvae  an  abnormal  growth 
of  the  plant  occurs  about  the 
young  insect,  forming  an  in- 
closing gall  that  serves  not  only 
to  protect  the  insect  within, 
but  to  furnish  it  with  an  abun- 
dance of  plant-sap,  its  food.  The 
young  insect  remains  in  the  gall 
until  it  completes  its  develop- 
ment and  growth,  when  it 
gnaws  its  way  out.  Such  insect  galls  are  especially  abun- 
dant on  oak  trees  (Fig.  80).  The  care  of  the  eggs  and  the 
young  of  the  social  insects,  as  the  bees  and  ants,  are  de- 
scribed in  Chapter  IX. 


FIG.  79.— The  cottony  cushion  scale 
insect  (Icerya  purchasi),  from 
California.  The  male  is  winged, 
the  female  wingless  and  with  a 
large  waxen  egg-sac  (e.s.)  attached 
to  her  body.  (The  lines  at  the  left 
of  each  figure  indicate  the  size  of 
the  insects.) 


ADAPTATIONS  143 

80.  Adaptations  concerned  with  surroundings  in  life. — A 

large  part  of  the  life  of  the  animal  is  a  struggle  with  the 
environment  itself;  in  this  struggle  only  those  that  are 
adapted  live  and  leave  descendants  fitted  like  themselves. 
The  fur  of  mammals  fits  them  to  their  surroundings.  As 
the  fur  differs,  so  may  the  habits  change.  Some  animals 
are  active  in  winter ;  others,  as  the  bear,  hibernate,  sleep- 
ing in  caves  or  hollow  trees  or  in  burrows  until  conditions 
are  favorable  for  their  activity.  Most  snakes  and  lizards 
hibernate  in  cold  weather.  In  the  swamps  of  Louisiana, 


FIG.  80.— The  giant  gall  of  the  white  oak  (California),  made  by  the  gall  insect  Andri- 
cus  calif ornicus.  The  gall  at  the  right  cut  open  to  show  tunnels  made  by  the 
insects  in  escaping  from  the  gall.— From  photograph. 

in  winter,  the  bottom  may  often  be  seen  covered  with  water 
snakes  lying  as  inert  as  dead  twigs.  Usually,  however, 
hibernation  is  accompanied  by  concealment.  Some  animals 
in  hibernation  may  be  frozen  alive  without  apparent  injury. 
The  blackfish  of  the  Alaska  swamps,  fed  to  dogs  when 
frozen  solid,  has  been  known  to  revive  in  the  heat  of  the 
•dog's  stomach  and  to  wriggle  out  and  escape.  As  animals 
resist  heat  and  cold  by  adaptations  of  structure  or  habits, 
so  may  they  resist  dryness.  Certain  fishes  hold  reservoirs 


144 


ANIMAL  LIFE 


of  water  above  their  gills,  by  means  of  which  they  can 
breathe  during  short  excursions  from  the  water.  Still 
others  (mud-fishes)  retain  the  primitive  lung-like  structure 
of  the  swim-bladder,  and  are  able  to  breathe  air  when,  in  the 
dry  season,  the  water  of  the  pools  is  reduced  to  mud. 

Another  series  of  adaptations  is  concerned  with  the 
places  chosen  by  animals  for  their  homes.  The  fishes  that 
live  in  water  have  special  organs  for 
breathing  under  water  (Fig.  82). 
Many  of  the  South  American  mon- 
keys have  the  tip  of  the  tail  adapted 
for  clinging  to  limbs  of  trees  or  to 
the  bodies  of  other  monkeys  of  its 
own  kind.  The  hooked  claws  of  the 
bat  hold  on  to  rocks,  the  bricks  of 
chimneys,  or  to  the  surface  of  hollow 
trees  where  the  bat  sleeps  through 
the  day.  The  tree-frogs  (Fig.  83)  or 
tree-toads  have  the  tips  of  the  toes 
swollen,  forming  little  pads  by  which 
they  cling  to  the  bark  of  trees. 

Among  other  adaptations  relat- 
ing to  special  surroundings  or  con- 
ditions of  life  are  the  great  cheek 
pouches  of  the  pocket  gophers, 
which  carry  off  the  soil  dug  up  by 
the  large  shovel-like  feet  when  the 
gopher  excavates  its  burrow. 

Those  insects  which  live  under- 
ground, making  burrows  or  tunnels 
in  the  soil,  have  their  legs  or  other  parts  adapted  for  dig- 
ging and  burrowing.  The  mole  cricket  (Fig.  84)  has  its 
legs  stout  and  short,  with  broad,  shovel-like  feet.  Some 
water-beetles  (Fig.  85)  and  water-bugs  have  one  or  more  of 
the  pairs  of  legs  flattened  and  broad  to  serve  as  oars  or  pad. 
dies  for  swimming.  The  grasshoppers  or  locusts,  who  leap,. 


FIG.  81.— Insect  galls  on  leaf. 


ADAPTATIONS 


145 


have  their  hind  legs  greatly  enlarged  and  elon- 
gated, and  provided  with  strong  muscles,  so  as 
to  make  of  them  "leaping  legs."     The  grubs 


FIG.  82.— Head  of  rainbow  trout  (Salmo  irideus)  with  gill  cover  bent  back  to  show 
gills,  the  breathing  organs. 

or  larvae  of  beetles  which  live  as  "  borers "  in  tree-trunks 
have  mere  rudiments  of  legs,  or  none  at  all  (Fig.  86). 
They  have  great,  strong,  biting  jaws  for  cutting  away 
the  hard  wood.  They  move"  simply  by  wriggling  along 
in  their  burrows  or  tunnels. 

Insects    that    live 
in  water  either  come      - 
up  to  the  surface  to 
breathe  or  take  down 
air   underneath   their 
wings,     or     in     some 
other    way,    or    have 
gills  for  breathing  the 
air    which    is    mixed 
with  the  water.    These 
gills  are  special  adap- 
tive structures  which  present  a  great  variety  of  form  and 
appearance.    In  the  young  of  the  May-flies  they  are  deli- 
cate plate-like  flaps  projecting  from  the  sides  of  the  body. 
They  are  kept  in  constant  motion,  gently  waving  back  and 
11 


FIG.  83.— Tree-toad  (Hyla  regilla). 


146 


ANIMAL  LIFE 


forth  in  the  water  so  as  to  maintain  currents  to  bring  fresh 
water  in  contact  with  them.     Young  mosquitoes  (Fig.  87) 

do  not  have  gills,  but  come 
up  to  the  surface  to  breathe. 
The  larvae,  or  wrigglers, 
breathe  through  a  'special 


FIG.  84.— The  mole  cricket  (Gryllotalpa), 
with  fore  feet  modified  for  digging. 


FIG.  85.— A  water-beetle  (Hydroph- 

ilus). 


tube  at  the  posterior  tip  of  the  body,  while  the  pupae  have 
a  pair  of  horn-like  tubes  on  the  back  of  the  head  end  of 
the  body. 

81.  Degree  of  structural  change  in  adaptations.— While 
among  the  higher  or  vertebrate  animals,  especially  the 
fishes  and  reptiles,  most  remarkable  cases  of  adaptations 
occur,  yet  the  structural  changes  are  for  the  most  part  ex- 
ternal, never  seriously  affecting  the  development  of  the 
internal  organs  other 
than  the  skeleton.  The 
organization  of  these 
higher  animals  is  much 
less  plastic  than  among 
the  invertebrates.  In 
general,  the  higher  the  type  the  more  persistent  and  un- 
changeable are  those  structures  not  immediately  exposed 


FIG.  86.— Wood-boring  beetle  larva  (Prionus). 


ADAPTATIONS 


147 


to  the  influence  of  the  struggle  for  existence.  It  is  thus 
the  outside  of  an  animal  that  tells  where  its  ancestors 
have  lived.  The  inside,  suffering  little  change,  whatever 
the  surroundings,  tells  the  real  nature  of  the  animal. 

82.  Vestigial  organs. — In  general,  all  the  peculiarities  of 
animal  structure  find  their  explanation  in  some  need  of 
adaptation.  When  this  need  ceases,  the  structure  itself 
tends  to  disappear  or  else  to  serve  some  other  need.  In 
the  bodies  of  most  animals  there  are  certain  incomplete 
or  rudimentary  organs 
or  structures  which 
serve  no  distinct  use- 
ful purpose.  They  are 
structures  which,  in  the 
ancestors  of  the  ani- 
mals now  possessing 
them,  were  fully  devel- 
oped functional  organ.;, 
but  which,  because  of  u 
change  in  habits  or  con- 
ditions of  living,  are  of 
no  further  need,  and 
are  gradually  dying  out. 
Such  organs  are  called 
vestigial  organs.  Ex- 
amples are  the  disused 
ear  muscles  of  man,  the 
vermiform  appendix  in 
man,  .which  is  the  reduced  and  now  useless  anterior  end 
of  the  large  intestine.  In  the  lower  animals,  the  thumb  or 
degenerate  first  finger  of  the  bird  with  its  two  or  three  little 
quills  serves  as  an  example.  So  also  the  reduced  and  elevated 
hind  toe  of  certain  birds,  the  splint  bones  or  rudimentary 
side  toes  of  the  horse,  the  rudimentary  eyes  of  blind  fishes, 
the  minute  barbel  or  beard  of  the  horned  dace  or  chub,  and 
the  rudimentary  teeth  of  the  right  whales  and  sword-fish. 


FIG.    87. — Young   stages   of   the   mosquito, 
a,  larva  (wriggler) ;  b,  pupa. 


148 


ANIMAL   LIFE 


Each  of  these  vestigial  organs  tells  a  story  of  some  past 
adaptation  to  conditions,  one  that  is  no  longer  needed  in 
the  life  of  the  species.  They  have  the  same  place  in  the 
study  of  animals  that  silent  letters  have  in  the  study  of 
words.  For  example,  in  our  word  knight  the  Jc  and  gh  are 
no  longer  sounded ;  but  our  ancestors  used  them  both,  as 
the  Germans  do  to-day  in  their  cognate  word  'Knecht.  So 
with  the  French  word  temps,  which  means  time,  in  which 
both  p  and  s  are  silent.  The  Eomans,  from  whom  the 
French  took  this  word,  needed  all  its  letters,  for  they  spelled 
and  pronounced  it  tempus.  In  general,  every  silent  letter 
in  every  word  was  once  sounded.  In  like  manner,  every 
vestigial  structure  was  once  in  use  and  helpful  or  necessary 
to  the  life  of  the  animal  which  possessed  it. 


Borne  of  two  male  deer  interlocked  while  fighting.    Permission  of  G.  O.  SHIELDS, 
publisher  of  Recreation. 


CHAPTEE  IX 

ANIMAL   COMMUNITIES   AND   SOCIAL   LIFE 

83.  Man  not  the  only  social  animal— Man  is  commonly 
called  the  social  animal,  but  he  is  not  the  only  one  to 
which  this  term  may  be  applied.     There  are  many  others 
which  possess   a   social   or  communal  life.     A  moment's 
thought  brings  to  mind  the  familiar  facts  of  the  communal 
life  of  the  honey-bee  and  of  the  ants.    And  there  are  many 
other  kinds  of  animals,  not  so  well  known  to  us,  that  live 
in  communities  or  colonies,  and  live  a  life  which  in  greater 
or  less  degree  is  communal  or  social.     In  this  connection 
we  may  use  the  term  communal  for  the  life  of  those  ani- 
mals in  which  the  division  of  labor  is  such  that  the  indi- 
vidual is  dependent  for  its  continual  existence  on  the  com- 
munity as  a  whole.     The  term  social  life  would  refer  to  a 
lower  degree  of  mutual  aid  and  mutual  dependence. 

84.  The  honey-bee. — Honey-bees    live    together,  as  we 
know,  in  large  communities.     We  are  accustomed  to  think 
of  honey-bees  as  the  inhabitants  of  bee-hives,  but  there 
were  bees   before   there  were  hives.     The   "bee-tree"  is 
familiar  to  many  of  us.     The  bees,  in  Mature,  make  their 
home  in  the  hollow  of  some  dead  or  decaying  tree-trunk, 
and  carry  on  there  all  the  industries  which  characterize 
the  busy  communities  in  the   hives.     A  honey-bee   com- 
munity comprises  three  kinds  of  individuals  (Fig.  88) — 
namely,   a  fertile   female    or   queen,   numerous  males   or 
drones,   and  many  infertile   females  or  workers.     These 
three  kinds  of  individuals  diifer  in  external  appearance 
sufficiently  to  be  readily  recognizable.     The  workers  are 

11  149 


150  ANIMAL  LIFE 

smaller  than  the  queens  and  drones,  and  the  last  two  differ 
in  the  shape  of  the  abdomen,  or  hind  body,  the  abdomen  of 
the  queen  being  longer  and  more  slender  than  that  of  the 


FIG.  88.— Honey-bee,    a,  drone  or  male  ;  b,  worker  or  infertile  female  ;  c,  queen  or 
fertile  female. 

male  or  drone.  In  a  single  community  there  is  one  queen, 
a  few  hundred  drones,  and  ten  to  thirty  thousand  workers. 
The  number  of  drones  and  workers  varies  at  different 
times  of  the  year,  being  smallest  in  winter.  Each  kind  of 
individual  has  certain  work  or  business  to  do  for  the  whole 
community.  The  queen  lays  all  the  eggs  from  which  new 
bees  are  born;  that  is,  she  is  the  mother  of  the  entire 
community.  The  drones  or  males  have  simply  to  act  as 
royal  consorts ;  upon  them  depends  the  fertilization  of  the 
eggs.  The  workers  undertake  all  the  food-getting,  the 
care  of  the  young  bees,  the  comb-building,  the  honey-mak- 
ing— all  the  industries  with  which  we  are  more  or  less 
familiar  that  are  carried  on  in  the  hive.  And  all  the 
work  done  by  the  workers  is  strictly  work  for  the  whole ' 
community ;  in  no  case  does  the  worker  bee  work  for  itself 
alone ;  it  works  for  itself  only  in  so  far  as  it  is  a  member 
of  the  community. 

How  varied  and  elaborately  perfected  these  industries 
are  may  be  perceived  from  a  brief  account  of  the  life  his- 
tory of  a  bee  community.  The  interior  of  the  hollow  in 
the  bee-tree  or  of  the  hive  is  filled  with  "  comb  " — that  is, 
with  wax  molded  into  hexagonal  cells  and  supports  for 
these  cells.  The  molding  of  these  thousands  of  symmet- 


ANIMAL  COMMUNITIES  AND  SOCIAL  LIFE 


rical  cells  is  accomplished  by  the  workers  by  means  of  their 
specially  modified  trowel-like  mandibles  or  jaws.  The  wax 
itself,  of  which  the  cells  are  made,  comes  from  the  bodies 
of  the  workers  in  the  form  of  small 
liquid  drops  which  exude  from  the  skin 
on  the  under  side  of  the  abdomen  or 
hinder  body  rings.  These  droplets 
run  together,  harden  and  become  flat- 
tened, and  are  removed  from  the  wax 
plates,  as  the  peculiarly  modified  parts 
of  the  skin  which  produce  the  wax 
are  called,  by  means  of  the  hind  legs, 
which  are  furnished  with  scissor-like 
contrivances  for  cutting  off  the  wax 
(Fig.  89).  In  certain  of  the  cells  are 
stored  the  pollen  and  honey,  which 
serve  as  food  for  the  community.  The 
pollen  is  gathered  by  the  workers  from 
certain  favorite  flowers  and  is  carried 
by  them  from  the  flowers  to  the  hive 
in  the  "pollen  baskets,"  the  slightly 
concave  outer  surfaces  of  one  of  the 
segments  of  the  broadened  and  flattened 
hind  legs.  This  concave  surface  is  lined 
on  each  margin  with  a  row  of  incurved 

i*<w  i     •  1*11     IT     11  -n  FIG.  89. — Posterior  leg  of 

Stiff  hairs  which   hold    the   pollen    maSS       W0rker  honey-bee.  The 

securely  in  place  (Fig.  89).  The  "  honey  " 

is  the  nectar  of  flowers  which  has  been 

sucked  up  by  the  workers  by  means  of 

their    elaborate   lapping   and     sucking 

mouth  parts  and  swallowed  into  a  sort 

of  honey-sac  or  stomach,  then  brought 

to  the  hive  and  regurgitated  into  the 

cells.      This    nectar  is   at  first  too   watery  to    be    good 

honey,  so  the  bees  have  to  evaporate  some  of  this  water. 

Many  of   the  workers  gather  above   the   cells   containing 


concave  surface  of  the 
upper  large  joint  with 
the  marginal  hairs  is 
the  pollen  basket ;  the 
wax  shears  are  the  cut- 
ting surfaces  of  the 
angle  between  the  two 
large  segments  of  the 
leg. 


152  ANIMAL  LIFE 

nectar,  and  buzz — that  is,  vibrate  their  wings  violently. 
This  creates  currents  of  air  which  pass  over  the  exposed 
nectar  and  increase  the  evaporation  of  the  water.  The 
violent  buzzing  raises  the  temperature  of  the  bees'  bodies, 
and  this  warmth  given  off  to  the  air  also  helps  make  evap- 
oration more  rapid.  In  addition  to  bringing  in  food  the 
workers  also  bring  in,  when  necessary,  "  propolis,"  or  the 
resinous  gum  of  certain  trees,  which  they  use  in  repairing 
the  hive,  as  closing  up  cracks  and  crevices  in  it. 

In  many  of  the  cells  there  will  be  found,  not  pollen  or 
honey,  but  the  eggs  or  the  young  bees  in  larval  or  pupal 

condition  (Fig.  90). 
The  queen  moves 
about  through  the 
hive,  laying  eggs. 
She  deposits  only  one 
egg  in  a  cell.  In 
three  days  the  egg 
hatches,  and  the 
young  bee  appears 
as  a  helpless,  soft, 
white,  footless  grub 

FIG.  90.— Cells  containing  eggs,  larvae,  and  pupae  of  or  larva.  It  is  Cared 
the  honey-bee.  The  lower  large,  irregular  cells  -  •,  oortain  nf  ihp 
are  queen  cells.-After  BENTON. 

workers,  that  may  be 

called  nurses.  These  nurses  do  not  differ  structurally  from 
the  other  workers,  but  they  have  the  special  duty  of  caring 
for  the  helpless  young  bees.  They  do  not  go  out  for  pollen 
or  honey,  but  stay  in  the  hive.  They  are  usually  the  new 
bees — i.  e.,  the  youngest  or  most  recently  added  workers. 
After  they  act  as  nurses  for  a  week  or  so  they  take  their 
places  with  the  food-gathering  workers,  and  other  new 
bees  act  as  nurses.  The  nurses  feed  the  young  or  larval 
bees  at  first  with  a  highly  nutritious  food  called  bee-jelly, 
which  the  nurses  make  in  their  stomach,  and  regurgitate 
for  the  larvae.  After  the  larvae  are  two  or  three  days  old 


ANIMAL  COMMUNITIES  AND  SOCIAL  LIFE         153 

they  are  fed  with  pollen  and  honey.  Finally,  a  small  mass 
of  food  is  put  into  the  cell,  and  the  cell  is  "  capped  "  or 
covered  with  wax.  The  larva,  after  eating  all  the  food,  in 
two  or  three  days  more  changes  into  a  pupa,  which  lies 
quiescent  without  eating  for  thirteen  days,  when  it  changes 
into  a  full-grown  bee.  The  new  bee  breaks  open  the  cap 
of  the  cell  with  its  jaws,  and  comes  out  into  the  hive,  ready 
to  take  up  its  share  of  the  work  for  the  community.  In  a 
few  cases,  however,  the  life  history  is  different.  The  nurses 
will  tear  down  several  cells  around  some  single  one,  and 
enlarge  this  inner  one  into  a  great  irregular  vase-shaped 
cell.  When  the  egg  hatches,  the  grub  or  larva  is  fed  bee- 
jelly  as  long  as  it  remains'  a  larva,  never  being  given  ordi- 
nary pollen  and  honey  at  all.  This  larva  finally  pupates, 
and  there  issues  from  the  pupa  not  a  worker  or  drone  bee, 
but  a  new  queen.  The  egg  from  which  the  queen  is  pro- 
duced is  the  same  as  the  other  eggs,  but  the  worker  nurses 
by  feeding  the  larva  only  the  highly  nutritious  bee-jelly 
make  it  certain  that  the  new  bee  shall  become  a  queen 
instead  of  a  worker.  It  is  also  to  be  noted  that  the  male 
bees  or  drones  are  hatched  from  eggs  that  are  not  ferti- 
lized, the  queen  having  it  in  her  power  to  lay  either  ferti- 
lized or  unfertilized  eggs.  From  the  fertilized  eggs  hatch 
larvaB  which  develop  into  queens  or  workers,  depending  on 
the  manner  of  their  nourishment;  from  the  unfertilized 
eggs  hatch  the  males. 

When  several  queens  appear  there  is  much  excitement 
in  the  community.  Each  community  has  normally  a  single 
one,  so  that  when  additional  queens  appear  some  rearrange- 
ment is  necessary.  This  rearrangement  comes  about  first 
by  fighting  among  the  queens  until  only  one  of  the  new 
queens  is  left  alive.  Then  the  old  or  mother  queen  issues 
from  the  hive  or  tree  folio-wed  by  many  of  the  workers. 
She  and  her  followers  fly  away  together,  finally  alighting 
on  some  tree  branch  and  massing  there  in  a  dense  swarm. 
This  is  the  familiar  phenomenon  of  "swarming."  The 


154 


ANIMAL  LIFE 


swarm  finally  finds  a  new  hollow  tree,  or  in  the  case  of  the 
hive-bee  (Fig.  91)  the  swarm  is  put  into  a  new  hive,  where 
the  bees  build  cells,  gather  food,  produce  young,  and  thus 


FIG.  91.— Hiving  a  swarm  of  honey-bees.    Photograph  by  S.  J.  HUNTER. 

found  a  new  community.  This  swarming  is  simply  an  emi- 
gration, which  results  in  the  wider  distribution  and  in  the 
increase  of  the  number  of  the  species.  It  is  a  peculiar  but 
effective  mode  of  distributing  and  perpetuating  the  species. 
There  are  many  other  interesting  and  suggestive  things 
which  might  be  told  of  the  life  in  a  bee  community :  how 
the  community  protects  itself  from  the  dangers  of  starva- 
tion when  food  is  scarce  or  winter  comes  on  by  killing  the 
useless  drones  and  the  immature  bees  in  egg  and  larval 
stage ;  how  the  instinct  of  home-finding  has  been  so  highly 
developed  that  the  worker  bees  go  miles  away  for  honey 
and  nectar,  flying  with  unerring  accuracy  back  to  the  hive ; 
of  the  extraordinarily  nice  structural  modifications  which 
adapt  the  bee  so  perfectly  for  its  complex  and  varied  busi- 
nesses ;  and  of  the  tireless  persistence  of  the  workers  until 


ANIMAL   COMMUNITIES  AND  SOCIAL  LIFE         155 

they  fall  exhausted  and  dying  in  the  performance  of  their 
duties.  The  community,  it  is  important  to  note,  is  a  per- 
sistent or  continuous  one.  The  workers  do  not  live  long, 
the  spring  broods  usually  not  over  two  or  three  months, 
and  the  fall  broods  not  more  than  six  or  eight  months; 
but  new  ones  are  hatching  while  the  old  ones  are  dying, 
and  the  community  as  a  whole  always  persists.  The  queen 
may  live  several  years,  perhaps  as  many  as  five.*  She  lays 
about  one  million  eggs  a  year. 

85.  The  ants. — There  are  many  species  of  ants,  two 
thousand  or  more,  and  all  of  them  live  in  communities  and 
show  a  truly  communal  life.  There  is  much  variety  of 
habit  in  the  lives  of  different  kinds  of  ants,  and  the  degree 
in  which  the  communal  or  social  life  is  specialized  or  elab- 
orated varies  much.  But  certain  general  conditions  pre- 
vail in  the  life  of  all  the  different  kinds  of  individuals — 


a 


FIG.  92.— Female  (a),  male  (6),  and  worker  (c)  of  an  ant  (Camponotus  sp.). 

sexually  developed  males  and  females  that  possess  wings, 
and  sexually  undeveloped  workers  that  are  wingless  (Fig. 
92).  In  some  kinds  the  workers  show  structural  differ- 

*  A  queen  bee  has  been  kept  ^ive  for  fifteen  years. 


156  ANIMAL  LIFE 

ences  among  themselves,  being  divided  into  small  workers, 
large  workers,  and  soldiers.  The  workers  are  not,  as  with 
the  bees,  all  infertile  females,  but  they  are  both  male  and 
female,  both  being  infertile.  Although  the  life  of  the  ant 
communities  is  much  less  familiar  and  fully  known  than 
that  of  the  bees,  it  is  even  more  remarkable  in  its  speciali- 
zations and  elaborateness.  The  ant  home,  or  nest,  or  formi- 
cary, is,  with  most  species,  a  very  elaborate  underground, 
many-storied  labyrinth  of  galleries  and  chambers.  Certain 
rooms  are  used  for  the  storage  of  food ;  certain  others  as 
"  nurseries  "  for  the  reception  and  care  of  the  young ;  and 
others  as  stables  for  the  ants'  cattle,  certain  plant-lice  or 
scale-insects  which  are  sometimes  collected  and  cared  for  by 
the  ants.  The  food  of  ants  comprises  many  kinds  of  vege- 
table and  animal  substances,  but  the  favorite  food,  or  "  na- 
tional dish,"  as  it  has  been  called,  is  a  sweet  fluid  which  is 
produced  by  certain  small  insects,  the  plant-lice  (Aphidae) 
and  scale-insects  (Coccidae).  These  insects  live  on  the  sap 
of  plants ;  rose-bushes  are  especially  favored  with  their  pres- 
ence. The  worker  ants  (and  we  rarely  see  any  ants  but 
the  wingless  workers,  the  winged  males  and  females  appear- 
ing out  of  the  nest  only  at  mating  time)  find  these  honey- 
secreting  insects,  and  gently  touch  or  stroke  them  with  their 
feelers  (antennae),  when  the  plant-lice  allow  tiny  drops  of 
the  honey  to  issue  from  the  body,  which  are  eagerly  drunk 
by  the  ants.  It  is  manifestly  to  the  advantage  of  the  ants 
that  the  plant-lice  should  thrive ;  but  they  are  soft-bodied, 
defenseless  insects,  and  readily  fall  a  prey  to  the  wander- 
ing predaceous  insects  like  the  lady-birds  and  aphis  lions. 
So  the  ants  often  guard  small  groups  of  plant-lice,  attack- 
ing, and  driving  away  the  would-be  ravagers.  When  the 
branch  on  which  the  plant-lice  are  gets  withered  and  dry, 
the  ants  have  been  observed  to  carry  the  plant-lice  care- 
fully to  a  fresh,  green  branch.  In  the  Mississippi  Valley  a 
certain  kind  of  plant-louse  lives  on  the  roots  of  corn.  Its 
eggs  are  deposited  in  the  ground  in  the  autumn  and  hatch 


ANIMAL  COMMUNITIES  AND  SOCIAL  LIFE         157 

the  following  spring  before  the  corn  is  planted.  Now,  the 
common  little  brown  ant  lives  abundantly  in  the  corn- 
fields, and  is  specially  fond  of  the  honey  secreted  by  the 
corn-root  plant-louse.  So,  when  the  plant-lice  hatch  in  the 
spring  before  there  are  corn  roots  for  them  to  feed  on,  the 
little  brown  ants  with  great  solicitude  carefully  place  the 
plant-lice  on  the  roots  of  a  certain  kind  of  knotweed  which 
grows  in  the  field,  and  protect  them  until  the  corn  ger- 
minates. Then  the  ants  remove  the  plant-lice  to  the  roots 
of  the  corn,  their  favorite  food  plant.  In  the  arid  lands  of 
New  Mexico  and  Arizona  the  ants  rear  their  scale-insects 
on  the  roots  of  cactus.  Other  kinds  of  ants  carry  plant 
lice  into  their  nests  and  provide  them  with  food  there. 
Because  the  ants  obtain  food  from  the  plant-lice  and  take 
care  of  them,  the  plant-lice  are  not  inaptly  called  the  ants' 
cattle. 

Like  the  honey-bees,  the  young  ants  are  helpless  little 
grubs  or  larvaa,  and  are  cared  for  and  fed  by  nurses.  The 
so-called  ants'  eggs,  little  white,  oval  masses,  which  we 
often  see  being  carried  in  the  mouths  of  ants  in  and  out  of 
an  ants'  nest,  are  not  eggs,  but  are  the  pupae  which  are 
being  brought  out  to  enjoy  the  warmth  and  light  of  the 
sun  or  being  taken  back  into  the  nest  afterward. 

In  addition  to  the  workers  that  build  the  nest  and  col- 
lect food  and  care  for  the  plant-lice,  there  is  in  many 
species  of  ants  a  kind  of  individuals  called  soldiers.  These 
are  wingless,  like  the  workers,  and  are  also,  like  the  work- 
ers, not  capable  of  laying  or  of  fertilizing  eggs.  It  is  the 
business  of  the  soldiers,  as  their  name  suggests,  to  fight. 
They  protect  the  community  by  attacking  and  driving 
away  predaceous  insects,  especially  other  ants.  The  ants 
are  among  the  most  warlike  of  insects.  The  soldiers  of  a 
community  of  one  species  of  ant  often  sally  forth  and 
attack  a  community  of  some  other  species.  If  successful 
in  battle  the  workers  of  the  victorious  community  take 
possession  of  the  food  stores  of  the  conquered  and  carry 


158  ANIMAL  LIFE 

them  to  their  own  nest.  Indeed,  they  go  even  further ;  they 
may  make  slaves  of  the  conquered  ants.  There  are  numer- 
ous species  of  the  so-called  slave-making  ants.  The  slave- 
makers  carry  into  their  own  nest  the  eggs  and  larvae  and 
pupae  of  the  conquered  community,  and  when  these  come 
to  maturity  they  act  as  slaves  of  the  victors — that  is,  they 
collect  food,  build  additions  to  the  nests,  and  care  for  the 
young  of  the  slave-makers.  This  specialization  goes  so  far 
in  the  case  of  some  kinds  of  ants,  like  the  robber-ant  of 
South  America  (Ecitori),  that  all  of  the  Eciton  workers  have 
become  soldiers,  which  no  longer  do  any  work  for  them- 
selves. The  whole  community  lives,  therefore,  wholly  by 
pillage  or  by  making  slaves  of  other  kinds  of  ants.  There 
are  four  kinds  of  individuals  in  a  robber-ant  community — 
winged  males,  winged  females,  and  small  and  large  wing- 
less soldiers.  There  are  many  more  of  the  small  soldiers 
than  of  the  large,  and  some  naturalists  believe  that  the  few 
latter,  which  are  distinguished  by  heads  and  jaws  of  great 
size,  act  as  officers.  On  the  march  the  small  soldiers  are 
arranged  in  a  long,  narrow  column,  while  the  large  soldiers 
are  scattered  along  on  either  side  of  the  column  and  appear 
to  act  as  sentinels  and  directors  of  the  army.  The  obser- 
vations made  by  the  famous  Swiss  students  of  ants,  Huber 
and  Forel,  and  by  other  naturalists,  read  like  fairy  tales, 
and  yet  are  the  well-attested  and  often  reobserved  actual 
phenomena  of  the  extremely  specialized  communal  and 
social  life  of  these  animals. 

86.  Other  communal  insects. — The  termites  or  white  ants 
(not  true  ants)  are  communal  insects.  Some  species  of 
termites  in  Africa  live  in  great  mounds  of  earth,  often 
fifteen  feet  high.  The  community  comprises  hundreds  of 
thousands  of  individuals,  which  are  of  eight  kinds  (Fig  93), 
viz.,  sexually  active  winged  males,  sexually  active  winged 
females,  other  fertile  males  and  females  which  are  wingless, 
wingless  workers  of  both  sexes  not  capable  of  reproduc- 
tion, and  wingless  soldiers  of  both  sexes  also  incapable  of 


ANIMAL   COMMUNITIES  AND  SOCIAL  LIFE 


159 


reproduction.  The  production  of  new  individuals  is  the 
sole  business  of  the  fertile  males  and  females  ;  the  workers 
build  the  nest  and  collect  food,  and  the  soldiers  protect  the 
community  from  the  attacks  of  marauding  insects.  The 
queen  grows  to  monstrous  size,  being  sometimes 


FIG.  93. — Termites,    a,  queen  ;   6,  male ;  c,  worker ;  d,  soldier. 

five  or  six  inches  long,  while  the  other  individuals  of  the 
community  are  not  more  than  half  or  three  quarters  of 
an  inch  long.  The  great  size  of  the  queen  is  due  to  the 
enormous  number  of  eggs  in  her  body. 

The  bumble-bees  live  in  communities,  but  their  social 
arrangements  are  very  simple  ones  compared  with  those  of 
the  honey-bee.  There  is,  in  fact,  among  the  bees  a  series 
of  gradations  from  solitary  to  communal  life.  The  inter- 
esting little  green  carpenter-bees  live  a  truly  solitary  life. 
Each  female  bores  out  the  pith  from  five  or  six  inches  of 
an  elder  branch  or  raspberry  cane,  and  divides  this  space 
into  a  few  cells  by  means  of  transverse  partitions  (Fig.  94). 
In  each  cell  she  lays  an  egg,  and  puts  with  it  enough  food 
— flower  pollen — to  last  the  grub  or  larva  through  its  life. 


160 


ANIMAL  LIFE 


She  then  waits  in  an  upper  cell  of  the  nest  until  the  young 
bees  issue  from  their  cells,  when  she  leads  them  off,  and 
each  begins  active  life  on  its  own  account.  The  mining- 


FIG.  94.— Nest  of  carpenter-bee. 


FIG.  95.— Nest  of  Andrena,  the  mining-bee. 


bees  (Andrena),  which  make  little  burrows  (Fig.  95)  in  a 
clay  bank,  live  in  large  colonies — that  is,  they  make  their 
nest  burrows  close  together  in  the  same  clay  bank,  but  each 
female  makes  her  own  burrow,  lays  her  own  eggs  in  it,  fur- 
nishes it  with  food — a  kind  of  paste  of  nectar  and  pollen — 
and  takes  no  further  care  of  her  young.  Nor  has  she  at 
any  time  any  special  interest  in  her  neighbors.  But  with 
the  smaller  mining-bees,  belonging  to  the  genus  Halictus^ 
several  females  unite  in  making  a  common  burrow,  after 
which  each  female  makes  side  passages  of  her  own,  extend- 


ANIMAL  COMMUNITIES  AND  SOCIAL  LIFE 


161 


ing  from  the  main  or  public  entrance  burrow.  As  a  well- 
known  entomologist  has  said,  Andrena  builds  villages  com- 
posed of  individual  *  homes,  while  Halictus  makes  cities 
composed  of  apartment  houses.  The  bumble-bee  (Fig.  96), 
however,  establishes  a  real  community  with  a  truly  com- 
munal life,  although  a  very  simple  one.  The  few  bumble- 
bees which  we  see  in  winter  time  are  queens;  all  other 
bumble-bees  die  in  the  autumn.  In  the  spring  a  queen 
selects  some  deserted  nest  of  a  field-mouse,  or  a  hole  in 
the  ground,  gathers  pollen  which  she  molds  into  a  rather 
large  irregular  mass  and  puts  into 
the  hole,  and  lays  a  few  eggs  on  the 
pollen  mass.  The  young  grubs  or 
larvae  which  soon  hatch  feed  on  the 
pollen,  grow,  pupate,  and  issue  as 
workers — winged  bees  a  little  small- 
er than  the  queen.  These  workers 
bring  more  pollen,  enlarge  the  nest, 
and  make  irregular  cells  in  the  pol- 
len mass,  in  each  of  which  the  queen 
lays  an  egg.  She  gathers  no  more 
pollen,  does  no  more  work  except 
that  of  egg-laying.  From  these  new 
eggs  are  produced  more  workers,  and 
so  on  until  the  community  may  come 
to  be  pretty  large.  Later  in  the  sum- 
mer males  and  females  are  produced 
and  mate.  With  the  approach  of 
winter  all  the  workers  and  males  die, 
leaving  only  the  fertilized  females, 
the  queens,  to  live  through  the  win- 
ter and  found  new  communities  in 
the  spring. 

The  social  wasps  show  a  communal  life  like  that  of  the 
bumble-bees.     The  only  yellow-jackets  and  hornets  that 
live  through  the  winter  are  fertilized  females  or  queens. 
12 


FIG.  96.— Bumble-bees,  a, 
worker ;  ft,  queen  or  fer- 
tile female. 


162 


ANIMAL  LIFE 


When  spring  comes  each  queen  builds  a  small  nest  sus- 
pended from  a  tree  branch,  and  consisting  of  a  small  comb 
inclosed  in  a  covering  or  envelope  op'en  at  the  lower  end. 
The  nest  is  composed  of  "  wasp  paper,'7  made  by  chewing 
bits  of  weather-beaten  wood  taken  from  old  fences  or  out- 
buildings. In  each  of  the  cells  the  queen  lays  an  egg. 
She  deposits  in  the  cell  a  small  mass  of  food,  consisting  of 
some  chewed  insects  or  spiders.  From  these  eggs  hatch 
grubs  which  eat  the  food  prepared  for  them,  grow,  pupate, 
and  issue  as  worker  bees,  winged  and  slightly  smaller 
than  the  queen  (Fig.  97).  The  workers  enlarge  the  nest, 
adding  more  combs  and  making  many  cells,  in  each  of 
which  the  queen  lays  an  egg.  The  workers  provision  the 
cell  with  chewed  insects,  and  other  broods  of  workers  are 

rapidly  hatched.  The 
community  grows  in 
numbers  and  the  nest 
grows  in  size  until  it 
comes  to  be  the  great 
ball-like  oval  mass 
which  we  know  so  well 
as  a  hornets'  nest  (Figs. 
98  and  99),  a  thing  to  be 
left  untouched.  Some- 
times the  nest  is  built 
underground.  When 
disturbed,  they  swarm 
out  of  the  hole  and 
fiercely  attack  any  in- 
vading foe  in  sight. 
After  a  number  of 
broods  of  workers  has 
been  produced,  broods  of  males  and  females  appear  and 
mating  takes  place.  In  the  late  fall  the  males  and  all  of 
the  many  workers  die,  leaving  only  the  new  queens  to  live 
through  the  winter. 


FIG.  97.— The  yellow-jacket  (Vespa),  a  social 
wasp,    a,  worker  ;  &,  queen. 


ANIMAL  COMMUNITIES  AND  SOCIAL   LIFE         163 

The  bumble-bees  and  social  wasps  show  an  intermediate 
condition   between  the   simply   gregarious   or  neighborly 


FIG.  98.— Nest  of  Vespa,  a  social 
From  photograph. 


FIG.  99.— Nest  of   Vespa   opened  to  show 
combs  within. 


mining-bees  and  the  highly  developed,  permanent  honey- 
bee community.  Naturalists  believe  that  the  highly  or- 
ganized communal  life  of  the  honey-bees  and  the  ants  is 
a  development  from  some  simple  condition  like  that  of  the 
bumble-bees  and  social  wasps,  which  in  its  turn  has  grown 
out  of  a  still  simpler,  mere  gregarious  assembly  of  the 
individuals  of  one  species.  It  is  not  difficult  to  see  how 
such  a  development  could  in  the  course  of  a  long  time  take 
place. 

87.  Gregarionsness  and  mutual  aid. — The  simplest  form 
of  social  life  is  shown  among  those  kinds  of  animals  in 
which  many  individuals  of  one  species  keep  together,  form- 
ing a  great  band  or  herd.  In  this  case  there  is  not  much 
division  of  labor,  and  the  safety  of  the  individual  is  not 
wholly  bound  up  in  the  fate  of  the  herd.  Such  animals  are 


164  ANIMAL  LIFE 

said  to  be  gregarious  in  habit.  The  habit  undoubtedly  is 
advantageous  in  the  mutual  protection  and  aid  afforded 
the  individuals  of  the  band.  This  mutual  help  in  the  case 
of  many  gregarious  animals  is  of  a  very  positive  and  obvious 
character.  In  other  cases  this  gregariousness  is  reduced  to 
a  matter  of  slight  or  temporary  convenience,  possessing  but 
little  of  the  element  of  mutual  aid.  The  great  herds  of 
reindeer  in  the  north,  and  of  the  bison  or  buffalo  which 
once  ranged  over  the  Western  American  plains,  are  examples 
of  a  gregariousness  in  which  mutual  protection  from  ene- 
mies, like  wolves,  seems  to  be  th,e  principal  advantage  gained. 
The  bands  of  wolves  which  hunted  the  buffalo  show  the 
advantage  of  mutual  help  in  aggression  as  well  as  in  pro- 
tection. In  this  banding  together  of  wolves  there  is  active 
co-operation  among  individuals  to  obtain  a  common  food 
supply.  What  one  wolf  can  not  do — that  is,  tear  down  a 
buffalo  from  the  edge  of  the  herd — a  dozen  can  do,  and  all 
are  gainers  by  the  operation.  On  the  other  hand,  the  vast 
assembling  of  sea-birds  (Fig.  100)  on  certain  ocean  islands 
and  rocks  is  a  condition  probably  brought  about  rather  by 
the  special  suitableness  of  a  few  places  for  safe  breeding 
than  from  any  special  mutual  aid  afforded ;  still,  these  sea- 
birds  undoubtedly  combine  to  drive  off  attacking  eagles 
and  hawks.  Eagles  are  usually  considered  to  be  strictly 
solitary  in  habit  (the  unit  of  solitariness  being  a  pair,  not 
an  individual) ;  but  the  description,  by  a  Eussian  naturalist, 
of  the  hunting  habits  of  the  great  white-tailed  eagle  (Hali- 
cetos  alMcilla)  on  the  Russian  steppes  shows  that  this  kind 
of  eagle  at  least  has  adopted  a  gregarious  habit,  in  which 
mutual  help  is  plainly  obvious.  This  naturalist  once  saw  an 
eagle  high  in  the  air,  circling  slowly  and  widely  in  perfect 
silence.  Suddenly  the  eagle  screamed  loudly.  "  Its  cry 
was  soon  answered  by  another  eagle,  which  approached  it, 
and  was  followed  by  a  third,  a  fourth,  and  so  on,  till  nine 
or  ten  eagles  came  together  and  soon  disappeared."  The 
naturalist,  following  them,  soon  discovered  them  gathered 


ANIMAL  LIFE 

about  the  dead  body  of  a  horse.  The  food  found  by  the 
first  was  being  shared  by  all.  The  association  of  pelicans  in 
fishing  is  a  good  example  of  the  advantage  of  a  gregarious 
and  mutually  helpful  habit.  The  pelicans  sometimes  go 
fishing  in  great  bands,  and,  after  having  chosen  an  appro- 
priate place  near  the  shore,  they  form  a  wide  half-circle 
facing  the  shore,  and  narrow  it  by  paddling  toward  the 
land,  catching  the  fish  which  they  inclose  in  the  ever-nar- 
rowing circle. 

The  wary  Rocky  Mountain  sheep  (Fig.  101)  live  to- 
gether in  small  bands,  posting  sentinels  whenever  they 
are  feeding  or  resting,  who  watch  for  and  give  warning 
of  the  approach  of  enemies.  The  beavers  furnish  a  well- 
known  and  very  interesting  example  of  mutual  help,  and 
they  exhibit  a  truly  communal  life,  although  a  simple 
one.  They  live  in  "  villages  "  or  communities,  all  helping 
to  build  the  dam  across  the  stream,  which  is  necessary  to 
form  the  broad  marsh  or  pool  in  which  the  nests  or  houses 
are  built.  Prairie-dogs  live  in  great  villages  or  communi- 
ties which  spread  over  many  acres.  They  tell  each  other  by 
shrill  cries  of  the  approach  of  enemies,  and  they  seem  to 
visit  each  other  and  to  enjoy  each  other's  society  a  great 
deal,  although  that  they  afford  each  other  much  actual 
active  help  is  not  apparent.  Birds  in  migration  are  grega- 
rious, although  at  other  times  they  may  live  comparatively 
alone.  In  their  long  flights  they  keep  together,  often  with 
definite  leaders  who  seem  to  discover  and  decide  on  the 
course  of  flight  for  the  whole  great  flock.  The  wedge- 
shaped  flocks  of  wild  geese  flying  high  and  uttering  their 
sharp,  metallic  call  in  their  southward  migrations  are  well 
known  in  many  parts  of  the  United  States.  Indeed,  the 
more  one  studies  the  habits  of  animals  the  more  examples 
of  social  life  and  mutual  help  will  be  found.  Probably  most 
animals  are  in  some  degree  gregarious  in  habit,  and  in  all 
cases  of  gregariousness  there  is  probably  some  degree  of 
mutual  aid. 


FIG.  101.— Rocky  Mountain  or  bighorn  sheep.    By  permission  of  the 
publishers  of  Outing. 


168  ANIMAL  LIFE 

88.  Division  of  labor  and  basis  of  communal  life. — We  have 
learned  in  Chapters  II  and  IV  that  the  complexity  of  the 
bodies  of  the  higher  animals  depends  on  a  specialization  or 
differentiation  of  parts,  due  to  the  assumption  of  different 
functions  or  duties  by  different  parts  of  the  body ;  that  the 
degree  of  structural  differentiation  depends  on  the  degree 
or  extent  of  division  of  labor  shown  in  the  economy  of  the 
animal.  It  is  obvious  that  the  same  principle  of  division  of 
labor  with  accompanying  modification  of  structure  is  the 
basis  of  colonial  and  communal  life.  It  is  simply  a  mani- 
festation of  the  principle  among  individuals  instead  of 
among  organs.  The  division  of  the  necessary  labors  of  life 
among  the  different  zooids  of  the  colonial  jelly-fish  is  plain- 
ly the  reason  for  the  profound  and  striking,  but  always 
reasonable  and  explicable  modifications  of  the  typical  polyp 
or  medusa  body,  which  is  shown  by  the  swimming  zooids, 
the  feeding  zooids,  the  sense  zooids,  and  the  others  of  the 
colony.  And  similarly  in  the  case  of  the  termite  commu- 
nity, the  soldier  individuals  are  different  structurally  from 
the  worker  individuals  because  of  the  different  work  they 
have  to  do.  And  the  queen  differs  from  all  the  others,  be- 
cause of  the  extraordinary  prolificacy  demanded  of  her  to 
maintain  the  great  community. 

It  is  important  to  note,  however,  that  among  those  ani- 
mals that  show  the  most  highly  organized  or  specialized 
communal  or  social  life,  the  structural  differences  among 
the  individuals  are  the  least  marked,  or  at  least  are  not  the 
most  profound.  The  three  kinds  of  honey-bee  individuals 
differ  but  little;  indeed,  as  two  of  the  kinds,  male  and 
female,  are  to  be  found  in  the  case  of  almost  all  kinds  of 
animals,  whether  communal  in  habit  or  not,  the  only  unu- 
sual structural  specialization  in  the  case  of  the  honey-bee,  is 
the  presence  of  the  worker  individual,  which  differs  from 
the  usual  individuals"  In  but  little  more  than  the  rudimen- 
tary condition  of  the  reproductive  glands.  Finally,  in  the 
case  of  man,  with  whom  the  communal  or  social  habit  is  so 


170  ANIMAL  LIFE 

all-important  as  to  gain  for  him  the  name  of  "  the  social 
animal,"  there  is  no  differentiation  of  individuals  adapted 
only  for  certain  kinds  of  work.  Among  these  highest 
examples  of  social  animals,  the  presence  of  an  advanced 
mental  endowment,  the  specialization  of  the  mental  power, 
the  power  of  reason,  have  taken  the  place  of  and  made 
unnecessary  the  structural  differentiation  of  individuals. 
The  honey-bee  workers  do  different  kinds  of  work :  some 
gather  food,  some  care  for  the  young,  and  some  make  wax 
and  build  cells,  but  the  individuals  are  interchangeable; 
each  one  knows  enough  to  do  these  various  things.  There 
is  a  structural  differentiation  in  the  matter  of  only  one 
special  work  or  function,  that  of  reproduction. 

With  the  ants  there  is, 'in  some  cases,  a  considerable 
structural  divergence  among  individuals,  as  in  the  genus 
Atta  of  South  America  with  six  kinds  of  individuals — 
namely,  winged  males,  winged  females,  wingless  soldiers, 
and  wingless  workers  of  three  distinct  sizes.  In  the  case 
of  other  kinds  with  quite  as  highly  organized  a  communal 
life  there  are  but  three  kinds  of  individuals,  the  winged 
males  and  females  and  the  wingless  workers.  The  workers 
gather  food,  build  the  nest,  guard  the  "  cattle  "  (aphids), 
make  war,  and  care  for  the  young.  Each  one  knows  enough 
to  do  all  these  various  distinct  things.  Its  body  is  not  so 
modified  that  it  can  do  but  one  kind  of  thing,  which  thing 
it  must  always  do. 

The  increase  of  intelligence,  the  development  of  the 
power  of  reasoning,  is  the  most  potent  factor  in  the  devel- 
opment of  a  highly  specialized  social  life.  Man  is  the 
example  of  the  highest  development  of  this  sort  in  the  ani- 
mal kingdom,  but  the  highest  form  of  social  development 
is  not  by  any  means  the  most  perfectly  communal. 

89.  Advantages  of  communal  life. — The  advantages  of 
communal  or  social  life,  of  co-operation  and  mutual  aid,  are 
real.  The  animals  that  have  adopted  such  a  life  are  among 
the  most  successful  of  all  animals  in  the  struggle  for  exist- 


ANIMAL  COMMUNITIES  AND  SOCIAL  LIFE 

ence.  The  termite  individual  is  one  of  the  most  defense- 
less, and,  for  those  animals  that  prey  on  insects,  one  of 
the  most  toothsome  luxuries  to  be  found  in  the  insect 
world.  But  the  termite  is  one  of  the  most  abundant  and 
widespread  and  successfully  living  insect  kinds  in  all  the 
tropics.  Where  ants  are  not,  few  insects  are.  The  honey- 
bee is  a  popular  type  of  a  successful  life.  The  artificial 
protection  afforded  the  honey-bee  by  man  may  aid  in  its 
struggle  for  existence,  but  it  gains  this  protection  because 
of  certain  features  of  its  communal  life,  and  in  Nature  the 
honey-bee  takes  care  of  itself  well.  The  Little  Bee  People 
of  Kipling's  Jungle  Book,  who  live  in  great  communities  in 
the  rocks  of  Indian  hills,  can  put  to  rout  the  largest  and 
fiercest  of  the  jungle  animals.  Co-operation  and  mutual 
aid  are  among  the  most  important  factors  which  help  in 
the  struggle  for  existence.  It's  great  advantages  are,  how- 
ever, in  some  degree  balanced  by  the  fact  that  mutual  help 
brings  mutual  dependence.  The  community  or  society  can 
accomplish  greater  things  than  the  solitary  individuals,  but 
co-operation  limits  freedom,  and  often  sacrifices  the  indi- 
vidual to  the  whole. 


CHAPTEK  X 

COMMENSALISM   AND   SYMBIOSIS 

90.  Association  between  animals  of  different  species. — The 

living  together  and  mutual  help  discussed  in  the  last  chap- 
ter concerned  in  each  instance  a  single  species  of  animal. 
All  the  various  members  of  a  pack  of  wolves  or  of  a  com- 
munity of  ants  are  individuals  of  the  same  species.  But 
there  are  many  instances  of  an  association  of  individuals 
of  different  kinds  of  animals.  The  number  of  individuals 
concerned,  however,  is  usually  but  two — that  is,  one  of 
each  of  the  two  kinds  of  animals.  In  many  cases  of  an 
association  of  individuals  of  different  species  one  kind 
derives  great  benefit  and  the  other  suffers  more  or  less 
injury  from  the  association.  One  kind  lives  at  the  expense 
of  the  other.  This  association  is  called  parasitism,  and  is 
discussed  in  the  next  chapter.  In  some  cases,  however, 
neither  kind  of  animal  suffers  from  the  presence  of  the 
other.  The  two  live  together  in  harmony  and  presumably 
to  their  mutual  advantage.  In  some  cases  this  mutual 
advantage  is  obvious.  This  kind  of  association  is  called 
commensalism  or  symbiosis.  The  term  commensalism  may 
be  used  to  denote  a  condition  where  the  two  animals  are 
not  so  intimately  associated  nor  derive  such  obvious  mu- 
tual advantage  from  the  association,  as  in  that  condition 
of  very  intimate  and  permanent  association  with  obvious 
co-operative  and  marked  advantage  that  may  be  called 
symbiosis.  A  few  examples  of  each  of  these  interesting 
conditions  of  association  between  which  it  is  impossible  to 
make  any  sharp  distinction,  will  be  given. 
172 


COMMENSALISM  AND  SYMBIOSIS  173 

91.  Commensalism. — A  curious  example  of  commensalism 
is  afforded  by  the  different  species  of  Kemoras  (Echenididoe) 
which  attach  themselves  to  sharks,  barracudas,  and  other 
large  fishes  by  means  of  a  sucking  disk  on  the  top  of  the 
head  (Fig.  103).  This  disk  is  made  by  a  modification  of 


FIG.  103.—  Remora,  with  dorsal  fin  modified  to  be  a  sucking  plate  by  which  the 
fish  attaches  itself  to  a  shark. 

the  dorsal  fin.  The  Remora  thus  attached  to  a  shark  may 
be  carried  about  for  weeks,  leaving  its  host  only  to  secure 
food.  This  is  done  by  a  sudden  dash  through  the  water. 
The  Remora  injures  the  shark  in  no  way  save,  perhaps,  by 
the  slight  check  its  presence  gives  to  the  shark's  speed  in 
swimming. 

Whales,  similarly,  often  carry  barnacles  about  with 
them.  These  barnacles  are  permanently  attached  to  the 
skin  of  the  whale  just  as  they  would  be  to  a  stone  or 
wooden  pile.  Many  small  crustaceans,  annelids,  mollusks, 
and  other  invertebrates  burrow  into  the  substance  of  living 
sponges,  not  for  the  purpose  of  feeding  on  them,  but  for 
shelter.  On  the  other  hand,  the  little  boring  sponge 
(Cliona)  burrows  in  the  shells  of  oysters  and  other  bivalves 
for  protection.  These  are  hardly  true  cases  of  even  that 
lesser  degree  of  mutually  advantageous  association  which 
we  are  calling  commensalism.  But  some  species  of  sponge 
"  are  never  found  growing  except  on  the  backs  or  legs  of 
certain  crabs."  In  these  cases  the  sponge,  with  its  many 
plant-like  branches,  protects  the  crab  by  concealing  it  from 
its  enemies,  while  the  sponge  is  benefited  by  being  carried 
about  by  the  crab  to  new  food  supplies.  Certain  sponges 


ANIMAL  LIFE 

and  polyps  are  always  found  growing  in  close  association, 
though  what  the  mutual  advantage  of  this  association  is 
has  not  yet  been  found  out. 

Among  the  coral  reefs  near  Thursday  Island  (between 
New  Guinea  and  Australia)  there  lives  an  enormous  kind 
of  sea-anemone  or  polyp.  Individuals  of  this  great  polyp 
measure  two  feet  across  the  disk  when  fully  expanded. 
In  the  interior,  the  stomach  cavity,  which  communicates 
freely  with  the  outside  by  means  of  the  large  mouth  open- 
ing at  the  free  end  of  the  polyp,  there  may  often  be  found 
a  small  fish  (Amphiprion  percula).  That  this  fish  is  pur- 
posely in  the  gastral  cavity  of  the  polyp  is  proved  by  the 
fact  that  when  it  is  dislodged  it  invariably  returns  to  its 
singular  lodging-place.  The  fish  is  brightly  colored,  being 
of  a"  brilliant  vermilion  hue  with  three  broad  white  cross 
bands.  The  discoverer  of  this  peculiar  habit  suggests  that 
there  are  mutual  benefits  to  fish  and  polyp  from  this  habit. 
"  The  fish  being  conspicuous,  is  liable  to  attacks,  which  it 
escapes  by  a  rapid  retreat  into  the  sea-anemone ;  its  enemies 
in  hot  pursuit  blunder  against  the  outspread  tentacles  of 
the  anemone  and  are  at  once  narcotized  by  the  'thread 
cells '  shot  out  in  innumerable  showers  from  the  tentacles, 
and  afterward  drawn  into  the  stomach  of  the  anemone  and 
digested." 

Small  fish  of  the  genus  Nomeus  may  often  be  found 
accompanying  the  beautiful  Portuguese  man-of-war  (Phy- 
salia)  as  it  sails  slowly^about  on  the  ocean's  surface  (Fig. 
104).  These  little  fish  lurk  underneath  the  float  and 
among  the  various  hanging  thread-like  parts  of  the  Phy- 
sdlia,  which  are  provided  with  stinging  cells.  The  fish  are 
protected  from  their  enemies  by  their  proximity  to  these 
stinging  threads,  but  of  what  advantage  to  the  man-of- 
war  their  presence  is  is  not  understood.  Similarly,  several 
kinds  of  medusae  are  known  to  harbor  or  to  be  accompanied 
by  young  or  small  adult  fishes. 

In  the  nests  of  the  various  species  of  ants  and  termites 


COMMBNSALISM  AND 


IOSIS 


175 


•/i'-Vs 
(IFrAfrS 


(i' 


many  different  kinds  of  other  insects  have  been  found. 
Some  of  these  are  harmful  to  their  hosts,  in  that  they  feed 
on  the  food  stores  gathered  by  the  industrious  and  provi- 
dent ant,  but  others  appear 
to  feed  only  on  refuse  or  use- 
less substances  in  the  nest. 
Some  may  even  be  of  help  to 
their  hosts.  Over  one  thou- 
sand species  of  these  myrme- 
cophilous  (ant -loving)  and 
termitophilous  (termite  -  lov- 
ing) insects  have  been  re- 
corded by  collectors  as  living 
habitually  in  the  nests  of  ants 
and  termites.  The  owls  and 
rattlesnakes  which  live  with 
the  prairie-dogs  in  their  vil- 
lages afford  a  familiar  exam- 
ple of  commensalism. 

92.  Symbiosis.— Of  a  more 
intimate  character,  and  of 
more  obvious  and  certain  mu- 
tual advantage,  is  the  well- 
known  case  of  the  » symbiotic 
association  of  som,e  of  the 
numerous  species  of  hermit- 
crabs  and  certain  species  of 
sea-anemones.  The  hermit- 
crab  always  takes  for  his 
habitation  the  shell  of  an- 
other animal,  often  that  of 
the  common  whelk.  All  of 

the  hind  part  of  the  crab  lies  inside  the  shell,  while  its 
head  with  its  great  claws  project  from  the  opening  of  the 
shell.  On  the  surface  of  the  shell  near  the  opening  there 
is  often  to  be  found  a  sea-anemone,  or  sea-rose  (Fig.  105). 


FIG  104.— A  Portuguese  man-of-war 
(Physalia),  with  man-of-war  fishes 
(Nomws  gronovii)  living  in  the 
shelter  of  the  stinging  feelers. 
Specimens  from  off  Tampa,  Fla. 


176  ANIMAL  LIFE 

This  sea-anemone  is  fastened  securely  to  the  shell,  and  has 
its  mouth  opening  and  tentacles  near  the  head  of  the  crab. 
The  sea-anemone  is  carried  from  place  to  place  by  the  her- 
mit-crab, and  in  this  way  is  much  aided  in  obtaining  food. 
On  the  other  hand,  the  crab  is  protected  from  its  enemies 
by  the  well-armed  and  dangerous  tentacles  of  the  sea-anem- 


PIG.  105.— Hermit-crab  (Paguruf)  in  shell,  with  a  sea-anemone  (Adamsia  pattiatd) 
attached  to  the  shell.— After  HEBTWIG. 


one.  In  the  tentacles  there  are  many  thousand  long, 
slender  stinging  threads,  and  the  fish  that  would  obtain 
the  hermit-crab  for  food  must  first  deal  with  the  stinging 
anemone.  There  is  no  doubt  here  of  the  mutual  advan- 
tage gained  by  these  two  widely  different  but  intimately 
associated  companions.  If  the  sea-anemone  be  torn  away 
from  the  shell  inhabited  by  one  of  these  crabs,  the  crab 
will  wander  about,  carefully  seeking  for  another  anemone. 
When  he  finds  it  he  struggles  to  loosen  it  from  its  rock 
or  from  whatever  it  may  be  growing  on,  and  does  not  rest 
until  he  has  torn  it  loose  and  placed  it  on  his  shell. 

There  are  numerous  small  crabs  called  pea-crabs  (Pin- 
notheres) which  live  habitually  inside  the  shells  of  living 


COMMENSALISM  AND  SYMBIOSIS 


ITT 


mussels.     The  mussels  and  the  crabs  live  together  in  per- 
fect harmony  and  to  their  mutual  benefit. 

There  are  a  few  extremely  interesting  cases  of  symbiosis 
in  which  not  different  kinds  of  animals  are  concerned,  but 
animals  and  plants.     It  has  long  been  known  that  some 
sea-anemones  pos- 
sess  certain  body 
cells    which    con- 
tain    chlorophyll, 
that     green     sub- 
stance    character- 
istic of  the  green 
plants,   and     only 
in  few  cases  pos- 
sessed by  animals. 
When  these   chlo- 
rophyll -bearing 
sea-anemones  were 
first  found,  it  was 
believed  that  the 
chlorophyll     cells 
really  belonged  to 
the  animal's  body, 
and  that  this  con- 
dition broke  down  one  of  the  chiefest  and  most  readily 
apparent  distinctions  between  animals  and  plants.     But 
it  is  now  known  that  these  chlorophyll-bearing  cells  are 
microscopic,  one-celled  plants,  green  algae,  which  live  ha- 
bitually in  the  bodies  of  the  sea-anemone.     It  is  a  case 
of  true  symbiosis.     The  algae,  or  plants,  use  as  food  the 
carbonic-acid  gas  which  is  given   off  in   the   respiratory 
processes  of  the  sea-anemone,  and  the  sea-anemone  breathes 
in  the  oxygen  given  off  by  the  algae  in  the  process  of  ex- 
tracting the  carbon  for  food  from  the  carbonic-acid  gas. 
These  algae,  or  one-celled  plants,  lie  regularly  only  in  the 
innermost  of  the  three  cell  layers  which  compose  the  wall 
13 


FIG.  106.— The  crab  Epizoanthus  paguriphUus,  with 
the  sea-anemone  Parapagurus  pilosiramus  on  its 
shell. 


178 


ANIMAL  LIFE 


or  body  of  the  sea-anemone  (Fig.  107).      They  penetrate 
into  and  lie  in  the  interior  of  the  cells  'of  this  layer  whose 
special  function  is  that  of  digestion.    They  give  this  inner- 
most layer  of  cells 
a     distinct     green 
color. 

There  are  other 
examples  known  of 
the  symbiotic  asso- 
ciation of  plants 
and  animals ;  and 
if  we  were  to  fol- 
low the  study  of 
symbiosis  into  the 
plant  kingdom  we 
should  find  that  in 
one  of  the  large 
groups  of  plants, 
the  familiar  lichens 
which  grow  on 

rocks  and  tree  trunks  and  old  fences,  every  member  lives 
symbiotically.  A  lichen  is  not  a  single  plant,  but  is  always 
composed  of  two  plants,  an  alga  (chlorophyll-bearing)  and 
a  fungus  (without  chlorophyll)  living  together  in  a  most 
intimate,  mutually  advantageous  association. 


FIG.  107.— Diagrammatic  section  of  sea-anemone,  a, 
the  inner  cell  layer  containing  alga  cells,  the  two 
isolated  cells  at  right  being  cells  of  this  layer  with 
contained  algae ;  b,  middle  body  wall  layer;  c,  outer 
body  wall  layer.— After  HERTWIG. 


CHAPTEK  XI 

PARASITISM    AND   DEGENERATION 

93.  Relation  of  parasite  and  host. — In  addition  to  the  vari- 
ous ways  of  living  together  of  animals  already  described, 
namely,  the  social  life  of  individuals  of  a  single  species  and 
the  commensal  and  symbiotic  life  of  individuals  of  differ- 
ent species,  there  is  another  kind  of  association  among  ani- 
mals that  is  very  common.  In  cases  of  symbiosis  the  two 
animals  living  together  are  of  mutual  advantage  to  each 
other ;  both  profit  by  the  association.  But  there  are  many 
instances  in  the  animal  kingdom  of  an  association  between 
two  animals  by  which  one  gains  advantages  great  or  small, 
sometimes  even  obtaining  all  the  necessities  of  life,  while 
the  other  gains  nothing,  but  suffers  corresponding  disad- 
vantage, often  even  the  loss  of  life  itself.  This  is  the  asso- 
ciation of  parasite  and  host ;  the  relation  between  two  ani- 
mals whereby  one,  the  parasite,  lives  on  or  in  the  other,  the 
host,  and  at  the  expense  of  the  host.  Parasitism  is  a  com- 
mon phenomenon  in  all  groups  of  animals,  although  the 
parasites  themselves  are  for  the  most  part  confined  to  the 
classes  of  invertebrates.  Among  the  simplest  animals  or 
Protozoa  there  are  parasites,  as  Gregarina,  which  lives  in 
the  bodies  of  insects  and  crustaceans ;  there  are  parasitic 
worms,  and  parasitic  crustaceans  and  mollusks  and  insects, 
and  a  few  vertebrates.  When  an  animal  can  get  along 
more  safely  or  more  easily  by  living  at  the  expense  of  some 
other  animal  and  takes  up  such  a  life,  it  becomes  a  parasite. 
Parasitism  is  naturally,  therefore,  not  confined  to  any  one 
group  or  class  of  animals. 

179 


180  ANIMAL  LIFE 

94.  Kinds  of  parasitism.— The  bird-lice  (Mallophaga), 
which  infest  the  bodies  of  all  kinds  of  birds  and  are  found 
especially  abundant  on  domestic  fowls,  live  upon  the  out- 
side of  the  bodies  of  their  hosts,  feeding  upon  the  feathers 
and  dermal  scales.  They  are  examples  of  external  parasites. 
Other  examples  are  fleas  and  ticks,  and  the  crustaceans  called 
fish-lice  and  whale-lice,  which  are  attached  to  marine  ani- 
mals. On  the  other  hand,  almost  all  animals  are  infested  by 
certain  parasitic  worms  which  live  in  the  alimentary  canal, 
like  the  tape-worm,  or  imbedded  in  the  muscles,  like  the 
trichina.  These  are  examples  of  internal  parasites.  Such 
parasites  belong  mostly  to  the  class  of  worms,  and  some  of 
them  are  very  injurious,  sucking  the  blood  from  the  tissues 
of  the  host,  while  others  feed  solely  on  the  partly  digested 
food.  There  are  also  parasites  that  live  partly  within  and 
partly  on  the  outside  of  the  body,  like  the  Sacculina,  which 
lives  on  various  kinds  of  crabs.  The  body  of  the  Sacculina 
consists  of  a  soft  sac  which  lies  on  the  outside  of  the  crab's 
body,  and  of  a  number  of  long,  slender  root-like  processes 
which  penetrate  deeply  into  the  crab's  body,  and  take  up 
nourishment  from  within.  The  Sacculina  is  itself  a  crus- 
tacean or  crab-like  creature.  The  classification  of  para- 
sites as  external  and  internal  is  purely  arbitrary,  but  it  is 
often  a  matter  of  convenience. 

Some  parasites  live  for  their  whole  lifetime  on  or  in  the 
body  of  the  host,  as  is  the  case  with  the  bird-lice.  Their 
eggs  are  laid  on  the  feathers  of  the  bird  host ;  the  young 
when  hatched  remain  on  the  bird  during  growth  and  devel- 
opment, and  the  adults  only  rarely  leave  the  body,  usually 
never.  These  may  be  called  permanent  parasites.  On  the 
other  hand,  fleas  leap  off  or  on  a  dog  as  caprice  dictates ; 
or,  as  in  other  cases,  the  parasite  may  pass  some  definite 
part  of  its  life  as  a  free,  non-parasitic  organism,  attaching 
itself,  after  development,  to  some  animal,  and  remaining 
there  for  the  rest  of  its  life.  These  parasites  may  be  called 
temporary  parasites.  But  this  grouping  or  classification, 


PARASITISM  AND  DEGENERATION  181 

like  that  of  the  external  and  internal  parasites,  is  simply  a 
matter  of  convenience,  and  does  not  indicate  at  all  any 
blood  relationship  among  the  members  of  any  one  group. 

95.  The  simple  structure  of  parasites. — In  all  cases  the 
body  of  a  parasite  is  simpler  in  structure  than  the  body  of 
other  animals  which  are  closely  related  to  the  parasite — 
that  is,  animals  that  live  parasitically  have  simpler  bodies 
than  animals  that  live  free  active  lives,  competing  for 
food  with  the  other  animals  about  them.  This  simplicity 
is  not  primitive,  but  results  from  the  loss  or  atrophy  of  the 
structures  which  the  mode  of  life  renders  useless.  Many 
parasites  are  attached  firmly  to  their  host,  and  do  not  move 
about.  They  have  no  need  of  the  power  of  locomotion. 
They  are  carried  by  their  host.  Such  parasites  are  usually 
without  wings,  legs,  or  other  locomotory  organs.  Because 
they  have  given  up  locomotion  they  have  no  need  of  or- 
gans of  orientation,  those  special  sense  organs  like  eyes 
and  ears  and  feelers  which  serve  to  guide  and  direct  the 
moving  animal;  and  most  non-locomotory  parasites  will 
be  found  to  have  no  eyes,  nor  any  of  the  organs  of  special 
sense  which  are  accessory  to  locomotion  and  which  serve 
for  the  detection  of  food  or  of  enemies.  Because  these  im- 
portant organs,  which  depend  for  their  successful  activity 
on  a  highly  organized  nervous  system,  are  lacking,  the 
nervous  system  of  parasites  is  usually  very  simple  and  un- 
developed. Again,  because  the  parasite  usually  has  for 
its  sustenance  the  already  digested  highly  nutritious  food 
elaborated  by  its  host,  most  parasites  have  a  very  simple 
alimentary  canal,  or  even  no  alimentary  canal  at  all. 
Finally,  as  the  fixed  parasite  leads  a  wholly  sedentary  and 
inactive  life,  the  breaking  down  and  rebuilding  of  tissue  in 
its  body  go  on  very  slowly  and  in  minimum  degree,  and 
there  is  no  need  of  highly  developed  respiratory  and  circu- 
latory organs ;  so  that  most  fixed  parasites  have  these  sys- 
tems of  organs  in  simple  condition.  Altogether  the  body 
of  a  fixed,  permanent  parasite  is  so  simplified  and  so  want- 
13 


182  ANIMAL  LIFE 

ing  in  all  those  special  structures  which  characterize  the 
higher,  active,  complex  animals,  that  it  often  presents  a 
Very  different  appearance  from  those  animals  with  which 
we  know  it  to  be  nearly  related. 

The  simplicity  of  parasites  does  not  indicate  that  they 
all  belong  to  the  groups  of  primitive  simple  animals. 
Parasitism  is  found  in  the  whole  range  of  animal  life, 
from  primitive  to  highest.  Their  simplicity  is  something 
that  has  resulted  from  their  mode  of  life.  It  is  the  result 
of  a  change  in  the  body-structure  which  we  can  often 
trace  in  the  development  of  the  individual  parasite.  Many 
parasites  in  their  young  stages  are  free,  active  animals 
with  a  better  or  more  complex  body  than  they  possess  in 
their  fully  developed  or  adult  stage.  The  simplicity  of 
parasites  is  the  result  of  degeneration— a  degeneration 
that  has  been  brought  about  by  their  adoption  of  a  seden- 
tary, non-competitive  parasitic  life.  And  this  simplicity  of 
degeneration,  and  the  simplicity  of  primitiveness  should  be 
sharply  distinguished.  Animals  that  are  primitively  simple 
have  had  only  simple  ancestors ;  animals  that  are  simple 
by  degeneration  often  have  had  highly  organized,  complex 
ancestors.  And  while  in  the  life  history  or  development  of 
a  primitively  simple  animal  all  the  young  stages  are  simpler 
than  the  adult,  in  a  degenerate  animal  the  young  stages 
may  be,  and  usually  are,  more  complex  and  more  highly 
organized  than  the  adult  stage. 

In  the  examples  of  parasitism  that  are  described  in 
the  following  pages  all  these  general  statements  are  illus- 
trated. 

96.  Gregarina, — In  the  intestines  of  cray-fishes,  centi- 
peds,  and  several  kinds  of  insects  may  often  be  found 
certain  one-celled  animals  (Protozoa)  which  are  living  as 
parasites.  Their  food,  which  they  take  into  their  minute 
body  by  absorption,  is  the  intestinal  fluids  in  which  they 
lie.  These  parasitic  Protozoa  belong  to  the  genus  Grega- 
rina (Fig.  9)  (see  Chapter  I).  Because  the  body  of  any 


PARASITISM  AND  DEGENERATION  183 

protozoan  is  as  simple  as  an  animal's  body  can  be,  being 
composed  of  but  a  single  cell,  degeneration  can  not  occur 
in  the  cases  of  these  parasites.  There  are,  besides  Grega- 
rina,)  numerous  other  parasitic  one-celled  animals,  several 
kinds  living  inside  the  cells  of  their  host's  body.  One 
kind  lives  in  the  blood-corpuscles  of  the  frog,  and  another 
in  the  cells  of  the  liver  of  the  rabbit. 

97.  The  tape-worm  and  other  flat-worms.— In  the  great 
group  of  flat-worms  (Platyhelminthes),  that  group  of  ani- 
mals which  of  all  the  principal  animal  groups  is  widest 
in  its  distribution,  perhaps  a  major- 
ity of  the  species  are  parasites.  In- 
stead of  being  the  exception,  the 
parasitic  life  is  the  rule  among  these 
worms.  Of  the  three  classes  into 
which  the  flat -worms  are  divided 
almost  all  of  the  members  of  two  of 
the  classes  are  parasites.  The  com- 
mon tape-worm  (Tcenia)  (Fig.  108), 
which  lives  parasitically  in  the  intes- 
tine of  man,  is  a  good  example  of 
one  of  these  classes.  "  It  has  the 
form  of  a  narrow  ribbon,  which  may 
attain  the  length  of  several  yards, 
attached  at  one  end  to  the  wall  of  n0.io8.-Tape-worm<2te»ia 
the  intestine,  the  remainder  hanging  *#«!»).  in  upper  left- 

.        ,       .        .,        .     .       .        „      T,      ,  .  hand  corner  of  figure  the 

freely  in  the  interior."     Its  body  is       head  much  magnified.  - 
composed    of    segments    or    serially       After  LEUCKART. 
arranged  parts,  of  which  there  are 

about  eight  hundred  and  fifty  altogether.  It  has  no  mouth 
nor  alimentary  canal.  It  feeds  simply  by  absorbing  into 
its  body,  through  the  surface,  the  nutritious,  already  di- 
gested liquid  food  in  the  intestine.  There  are  no  eyes 
nor  other  special  sense  organs,  nor  any  organs  of  locomo- 
tion. The  body  is  very  degenerate.  The  life  history  of 
the  tape-worm  is  interesting,  because  of  the  necessity  of 


184 


ANIMAL  LIFE 


two  hosts  for  its  completion.  The  eggs  of  the  tape-worm 
pass  from  the  intestine  with  the  excreta,  and  must  be 
taken  into  the  body  of  some  other  animal  in  order  to  de- 
velop. In  the  case  of  one  of  the  several  species  of  tape- 
worms that  infest  man  this  other  host  must  be  the  pig. 
In  the  alimentary  canal  of  the  pig  the  young  tape-worm 
develops,  and  later  bores  its  way  through  the  walls  of  the 
canal  and  becomes  imbedded  in  the  muscles.  There  it  lies, 
until  it  finds  its  way  into  the '  alimentary  canal  of  man  by 
his  eating  the  flesh  of  the  pig.  In  the  intestine  of  man 
the  tape-worm  continues  to  develop 
until  it  becomes  full  grown. 

In  a  lake  in  Yellowstone  Park 
the  suckers  are  infested  by  one  of 
the  flat-worms  (Ligula)  that  at- 
tains a  size  of  nearly  one  fourth 
the  size  of  the  fish  in  whose  in- 
testines it  lives.  If  the  tape-worm 
of  man  attained  such  a  compara- 
tive size,  a  man  of  two  hundred 
pounds'  weight  would  be  infested  by 
a  parasite  of  fifty  pounds'  weight. 

98.  Trichina  and  other  round- 
worms. — Another  group  of  animals, 
many  of  whose  numbers  are  para- 
sites, are  the  round-worms  or  thread- 
worms (Nemathelminthes).  The 
free-living  round-worms  are  active, 
well  -  organized  animals,  but  the 
parasitic  kinds  all  show  a  greater 
or  less  degree  of  degeneration.  One 

of  the  most  terrible  parasites  of  man  is  a  round-worm  called 
Trichina  spiralis  (Fig.  109).  It  is  a  minute  worm,  from 
one  to  three  millimetres  long,  which  in  its  adult  condition 
lives  in  the  intestine  of  man  or  of  the  pig  or  other  mam- 
mals. The  young  are  born  alive  and  bore  through  the  walls 


FIG.  109.  —  Trichina  spiralis 
(after  GLAUS),  a,  male ;  ft, 
encysted  form  in  muscle  ;  c, 
female. 


PARASITISM  AND  DEGENERATION  185 

of  the  intestine.  They  migrate  to  the  voluntary  muscles 
of  the  hosts,  especially  those  of  the  limbs  and  back,  and 
here  each  worm  coils  itself  up  in  a  muscle  fiber  and  be- 
comes inclosed  in  a  spindle-shaped  cyst  or  cell  (Fig.  109,  V). 
A  single  muscle  may  be  infested  by  hundreds  of  thousands  of 
these  minute  worms.  It  has  been  estimated  that  fully  one 
hundred  million  encysted  worms  have  existed  in  the  mus- 
cles of  a  "  trichinized  "  human  body.  The  muscles  undergo 
more  or  less  degeneration,  and  the  death  of  the  host  may 
occur.  It  is  necessary,  for  the  further  development  of  the 
worms,  that  the  flesh  of  the  host  be  eaten  by  another  mam- 
mal, as  the  flesh  of  the  pig  by  man,  or  the  flesh  of  man  by 
a  pig  or  rat.  The  Trichinm  in  the  alimentary  canal  of 
the  new  host  develop  into  active  adult  worms  and  produce 
new  young. 

In  the  Yellowstone  Lake  the  trout  are  infested  by  the 
larvae  or  young  of  a  round-worm  (Bothriocephalus  cordiceps) 
which  reaches  a  length  of  twenty  inches,  and  which  is 
often  found  stitched,  as  it  were,  through  the  viscera  and 
the  muscles  of  the  fish.  The  infested  trout  become  feeble 
and  die,  or  are  eaten  by  the  pelicans  which  fish  in  this 
lake.  In  the  alimentary  canal  of  the  pelican  the  worms 
become  adult,  and  parts  of  the  worms  containing  eggs 
escape  from  the  alimentary  canal  with  the  excreta.  These 
portions  of  worms  are  eaten  by  the  trout,  and  the  eggs  give 
birth  to  new  worms  which  develop  in  the  bodies  of  the 
fish  with  disastrous  effects.  It  is  estimated  that  for  each 
pelican  in  Yellowstone  Lake  over  five  million  eggs  of  the 
parasitic  worms  are  discharged  into  the  lake. 

The  young  of  various  carnivorous  animals  are  often 
infested  by  one  of  the  species  of  round-worms  called  "  pup- 
worms  "  ( Uncinaria).  Recent  investigations  show  that 
thousands  of  the  young  or  pup  fur-seals  are  destroyed  each 
year  by  these  parasites.  The  eggs  of  the  worm  lie  through 
the  winter  in  the  sands  of  the  breeding  grounds  of  the  fur- 
seal.  The  young  receive  them  from  the  fur  of  the  mother 


PARASITISM  AND  DEGENERATION 


187 


and  the  worm  develops  in  the  upper  intestine.  It  feeds  on 
the  blood  of  the  young  seal,  which  finally  dies  from  anaemia. 
On  the  beaches  of  the  seal  islands  in  Bering  Sea  there  are 
sometimes  hundreds  of  dead  seal  pups  which  have  been 
killed  by  this  parasite  (Fig.  110). 

99.  Sacculina. — Among  the  more  highly  organized  ani- 
mals the  results  of  a  parasitic  life,  in  degree  of  structural 
degeneration,  can  be  more  readily  seen.  A  well-known  para- 
site, belonging  to  the  Crustacea — the  class  of  shrimps,  crabs, 
lobsters,  and  cray-fishes — is  Sacculina.  The  young  Sac- 
culina is  an  active,  free-swimming  larva  much  like  a  young 
prawn  or  young  crab.  But  the  adult  bears  absolutely  no 
resemblance  to  such  a  typical  crustacean  as  a  cray-fish  or 
crab.  The  Sacculina  after  a  short  period  of  independent 
existence  at- 
taches itself  to 
the  abdomen  of 
a  crab,  and 
there  completes 
its  develop- 
ment  while  liv- 
ing as  a  para- 
site. In  its 
adult  condition 
(Fig.  Ill)  it  is 
simply  a  great 
tumor-like  sac, 
bearing  many 
delicate  r  o  o  t- 

like  suckers  which  penetrate  the  body  of  the  crab  host  and 
absorb  nutriment.  The  Sacculina  has  no  eyes,  no  mouth 
parts,  no  legs,  or  other  appendages,  and  hardly  any  of  the 
usual  organs  except  reproductive  organs.  Degeneration 
here  is  carried  very  far. 

Other  parasitic   Crustacea,  as  the  numerous  kinds   of 
fish-lice  (Fig.  112)  which  live  attached  to  the  gills  or  to 


FIG.  ni.—Saccitlina,  a  crustacean  parasite  of  crabs,  a,  at- 
tached to  a  crab,  with  root-like  processes  penetrating  the 
crab's  body  ;  b,  removed  from  the  crab. 


188 


ANIMAL  LIFE 


other  parts  of  fish,  and  derive  all  their  nutriment  from  the 
body  of  the  fish,  show  various  degrees  of  degeneration.  With 
some  of  these  fish-lice  the  female, 
tyhich  looks  like  a  puffed-out  worm, 
is  attached  to  the  fish  or  other  aquatic 
animal,  while  the  male,  which  is  per- 
haps only  a  tenth  of  the  size  of  the 
female,  is  permanently  attached  to 
the  female,  living  parasitically  on  her. 
100.  Parasitic  insects.  —  Among 
the  insects  there  are  many  kinds 
that  live  parasitically  for  part  of 
their  life,  and  not  a  few  that  live  as 
parasites  for  their  whole  life.  The 
true  sucking  lice  (Fig.  113)  and  the 
bird-lice  (Fig.  114)  live  for  their  whole  lives  as  external 
parasites  on  the  bodies  of  their  host,  but  they  are  not 
fixed  —  that  is,  they  retain 
their  legs  and  power  of  loco- 
motion, although  they  have 
lost  their  wings  through  de- 
generation. The  eggs  of  the 
lice  are  deposited  on  the  hair 
of  the  mammal  or  bird  that 


FIG.   112. —  Fish-louse    (Ler- 
nceocerd).    a,  adult ;  b,  larva. 


FIG.  113.— Sucking  louse  (Pediculus)  of 
human  body. 


FIG.  114.— Bird  louse  (Lipewus  densus). 


serves  as  host ;  the  young  hatch  and  immediately  begin  to 
live  as  parasites,  either  sucking  the  blood  or  feeding  on  the 


PARASITISM  AND  DEGENERATION  189 

hair  or  feathers  of  the  host.  In  the  order  Hymenoptera 
there  are  several  families,  all  of  whose  members  live  during 
their  larval  stage  as  parasites.  We  may  call  all  these  hy- 
menopterous  parasites  ichneumon  flies.  The  ichneumon 
flies  are  parasites  of  other  insects,  especially  of  the  larvae  of 
beetles  and  moths  and  butterflies.  In  fact,  the  ichneumon 
flies  do  moro  to  keep  in  check  the  increase  of  injurious  and 
destructive  caterpillars  than  do  all  our  artificial  remedies 
for  these  insect  pests.  The  adult  'ichneumon  fly  is  four- 
winged  and  lives  an  active,  independent  life.  It  lays  its 
eggs  either  in  or  on  or  near  some  caterpillar  or  beetle  grub, 
and  the  young  ichneumon,  when  hatched,  burrows  about  in 
the  body  of  its  host,  feeding  on  its  tissues,  but  not  attacking 
such  organs  as  the  heart  or  nervous  ganglia,  whose  injury 
would  mean  immediate  death  to  the  host.  The  caterpillar 
lives  with  the  ichneumon  grub  within  it,  usually  until  nearly 


FIG.  115.— Parasitized  caterpillar  from  which  the    ichneumon    fly   parasites   have 
issued,  showing  the  circular  holes  of  exit  in  the  skin. 

time  for  its  pupation.  In  many  instances,  indeed,  it  pu- 
pates, with  the  parasite  still  feeding  within  its  body,  but  it 
never  comes  to  maturity.  The  larval  ichneumon  fly  pupates 
either  within  the  body  of  its  host  (Fig.  115)  or  in  a  tiny 
silken  cocoon  outside  of  its  body  (Fig.  116).  From  the 
cocoons  the  adult  winged  ichneumon  flies  emerge,  and 
after  mating  find  another  host  on  whose  body  to  lay  their 
eggs. 

One  of  the  most  interesting  ichneumon  flies  is  Thalessa 
(Fig.  119),  which  has  a  remarkably  long,  slender,  flexible 
ovipositor,  or  egg-laying  organ.  An  insect  known  as  the 


190  ANIMAL  LIFE 

pigeon  horn-tail  (Tremex  columba)  (Fig.  117)  deposits  its 
eggs,  by  means  of  a  strong,  piercing  ovipositor,  half  an  inch 
deep  in  the  trunk  wood  of  growing  trees.  The  young  or 


FIG.  116.— Caterpillar  with  cocoons  of  the  pupae  of  ichneumon  fly  parasites,  and 
(above)  one  of  the  adult  ichneumon  flies.    The  lines  indicate  natural  dimensions. 

larval  Tremex  is  a  soft-bodied  white  grub,  which  bores 
deeply  into  the  trunk  of  the  tree,  filling  up  the  burrow  be- 
hind it  with  small  chips.  The  Thalessa  is  a  parasite  of  the 
Tremex,  and  "  when  a  female  Thalessa  finds  a  tree  infested 
by  Tremex,  she  selects  a  place  which  she  judges  is  opposite 


PARASITISM  AND  DEGENERATION 


191 


a  Tremex  burrow,  and,  elevating  her  long  ovipositor  in  a 
loop  over  her  back,  with  its  tip  on  the  bark  of  the  tree  (Fig. 


FIG.   117.— The  pigeon   horn-tail  (Tremex 
columba),  with  strong  boring  ovipositor. 


FIG.  118.— Thalessa  lunator  boring,— After 
COMSTOCK. 


Fie.  -  HQ.^The  large  ichneumon  fly 
Thakssa,  with  long  flexible  oviposi- 
tor. The  various  parts  of  this  ovi- 
positor are  spread  apart  in  the  fig- 
ure ;  naturally  they  lie  together  to 
form  a  single  piercing  organ. 


118),  she  makes  a  derrick  out 

of  her  body  and  proceeds  with 

great  skill  and  precision  to  drill  a  hole  into  the  tree.    When 

the   Tremex  burrow  is  reached  she  deposits  an  egg  in  it. 


192 


ANIMAL  LIFE 


FIG.  120.— Wasp  (Polities),  with  female  Stylops  para- 
site (a;)  in  body. 


ism. 


The  larva  that  hatches  from  this  egg  creeps  along  this 
burrow  until  it  reaches  its  victim,  and  then  fastens  itself  to 
the  horn-tail  larva,  which  it  destroys  by  sucking  its  blood. 

The  larva  of  TJiales- 
sa,  when  full  grown, 
changes  to  a  pupa 
within  the  burrow 
of  its  host,  and  the 
adult  gnaws  a  hole 
out  through  the  bark 
if  it  does  not  find  the 
hole  already  made  by 
the  Tremex." 

The  beetles  of 
the  family  Stylopidae 
present  an  interest- 
ing case  of  parasit- 

The  adult  males  are  winged,  but  the  adult  females 
are  wingless  and  grub-like.  The  larval  stylopid  attaches 
itself  to  a  wasp  or  bee,  and  bores  into  its  abdomen.  It 
pupates  within  the  abdomen  of  the 
wasp  or  bee,  and  lies  there  with  its 
head  projecting  slightly  from  a  su- 
ture between  two  of  the  body  rings 
of  its  host  (Fig.  120).  The  adult 
finally  issues  and  leaves  the  host's 
body. 

Almost  all  of  the  mites  and  ticks, 
which  are  more  nearly  allied  to  the 
spiders  than  to  the  true  insects,  live 
parasitically.  Most  of  them  live  as 
external  parasites,  sucking  the  blood 
of  their  host,  but  some  live  under- 
neath the  skin  like  the  itch-mites 
(Fig.  121),  which  cause,  in  man,  the  disease  known  as 
the  itch. 


FIG.  121.— The  itch-mite 
(Sarcoptes  scabei). 


PARASITISM  AND  DEGENERATION  193 

101.  Parasitic  vertebrates. — Among  the  vertebrate  ani- 
mals there  are  not  many  examples  of  true  parasitism.     The 
hag-fishes  or  borers  (Myxine,  Heptatrema,  Polistotrema)  are 
long  and  cylindrical,  eel-like  creatures,  very  slimy  and  very 
low  in  structure.     The  mouth  is  without  jaws,  but  forms  a 
sucking  disk,  by  which  the  hag-fish  attaches  itself  to  the 
body  of  some  other  fish.     By  means  of  the  rasping  teeth  on 
its  tongue,  it  makes  a  round  hole  through  the  skin,  usually 
at  the  throat.     It  then  devours  all  the  muscular  substance 
of  the  fish,  leaving  the  viscera  untouched.     When  the  fish 
finally  dies  it  is  a  mere  hulk  of  skin,  scales,  bones,  and 
viscera,  nearly  all  the  muscle  being  gone.     Then  the  hag- 
fish  slips  out  and  attacks  another  individual. 

The  lamprey,  another  low  fish,  in  similar  fashion  feeds 
leech-like  on  the  blood  of  other  fishes,  which  it  obtains  by 
lacerating  the  flesh  with  its  rasp-like  teeth,  remaining  at- 
tached by  the  round  sucking  disk  of  its  mouth. 

Certain  birds,  as  the  cow-bird  and  the  European  cuckoo, 
have  a  parasitic .  habit,  laying  their  eggs  in  the  nests  of 
other  birds,  leaving,  their  young  to  be  hatched  and  reared 
by  their  unwilling  hosts.  This  is,  however,  not  bodily  para- 
sitism, such  as  is  seen  among  lower  forms. 

102.  Degeneration  through  quiescence.— While  parasitism 
is  the  principal  cause  of  degeneration  among  animals,  yet 
it  is  not  the  sole  cause.     It  is  evident  that  if  for  any  other 
reason  animals  should  become  fixed,  and  live  inactive  or 
sedentary  lives,  they  would  degenerate.     And  there  are  not 
a  few  instances  of  degeneration  due  simply  to  a  quiescent 
life,  unaccompanied  by  parasitism.     The  Tunicata,  or  sea^ 
squirts  (Fig.  122),  are  animals  which  have  become  simple 
through  degeneration,  due  to  the  adoption  of  a  sedentary 
life,  the  withdrawal  from  the  crowd  of  animals  and  from 
the  struggle  which  it  necessitates.     The  young  tunicate  is 
a  free-swimming,  active,  tadpole-like  or  fish-like  creature, 
which  possesses  organs  very  like  those  of  the  adult  of  the 
simplest  fishes  or  fish-like  forms.     That  is,  the  sea-squirt 

14 


194 


ANIMAL  LIFE 


begins  life  as  a  primitively  simple  vertebrate.     It 
in  its  larval  stage  a  notochord,-  the  delicate  structure  which 
precedes  the  formation  of  a  backbone,  extending  along  the 

upper  part  of  the  body, 
below  the  spinal  cord.  It 
is  found  in  all  young  vert 
tebrates,  and  is  chaYac- 
teristic  of  the  class.  The 
other  organs  of  the  young 
tunicate  are  all  of  verte- 
bral type.  But  the  young 
sea-squirt  passes  a  period 
of  active  and  free  life  as 
a  little  fish,  after  which 
it  settles  down  and  at- 
taches itself  to  a  stone  or 
shell  or  wooden  pier  by 
means  of  suckers,  and  re- 
mains for  the  rest  of  its 
life  fixed.  Instead  of  go- 
ing on  and  developing 
into  a  fish-like  creature,  it 
loses  its  notochord,  its 
special  sense  organs,  and 

other  organs ;  it  loses  its  complexity  and  high  organiza- 
tion, and  becomes  a  "  mere  rooted  bag  with  a  double  neck." 
a  thoroughly  degenerate  animal. 

A  barnacle  is  another  example  of  degeneration  through 
quiescence.  The  barnacles  are  crustaceans  related  most 
nearly  to  the  crabs  and  shrimps.  The  young  barnacle  just 
from  the  egg  (Fig.  123,  f)  is  a  six-legged,  free-swimming 
nauplius,  very  like  a  young  prawn  or  crab,  with  single  eye. 
In  its  next  larval  stage  it  has  six  pairs  of  swimming  feet, 
two  compound  eyes,  and  two  large  antennae  or  feelers,  and 
still  lives  an  independent,  free-swimming  life.  When  it 
makes  its  final  change  to  the  adult  condition,  it  attaches 


FIG.  122.— A  sea-squirt,  or  tunicate. 


PARASITISM  AND  DEGENERATION 


195 


itself  to  some  stone  or  shell,  or  pile  or  ship's  bottom,  loses 
its  compound  eyes  and  feelers,  develops  a  protecting  shell, 
and  gives  up  all  power  of  locomotion.  Its  swimming  feet 
become  changed  into  grasping  organs,  and  it  loses  most  of 
its  outward  resemblances  to  the  other  members  of  its  class 
(Fig.  123,  e). 


PIG.  123.— Three  adult  crustaceans  and  their  larvae,  a,  prawn  (Peneus),  active  and 
free-living ;  b,  larva  of  prawn  ;  c,  Sacculina,  parasite  ;  d,  larva  of  Sacculina ; 
e,  barnacle  (Lepas),  with  fixed  quiescent  life ;  /,  larva  of  barnacle.— After 
HAECKEL. 

Certain  insects  live  sedentary  or  fixed  lives.  All  the 
members  of  the  family  of  scale  insects  (Coccidae),  in  one 
sex  at  least,  show  degeneration,  that  has  been  caused  by 
quiescence.  One  of  these  coccids,  called  the  red  orange 
scale  (Fig.  124),  is  very  abundant  in  Florida  and  California 
and  in  other  orange-growing  regions.  The  male  is  a  beau- 
tiful, tiny,  two-winged  midge,  but  the  female  is  a  wingless, 


196 


ANIMAL  LIFE 


footless  little  sac  without  eyes  or  other  organs  of  special 
sense,  which  lies  motionless  under  a  flat,  thin,  circular,  red- 
dish scale  composed  of  wax  and  two  or  three  cast  skins  of 
the  insect  itself.  The  insect  has  a  long,  slender,  flexible, 
sucking  beak,  which  is  thrust  into  the  leaf  or  stem  or  fruit 
of  the  orange  on  which  the  "  scale  bug  "  lives  and  through 
which  the  insect  sucks  the  orange  sap,  which  is  its  only 


FIG.  124.- The  red  orange  scale  of  California,    o,  bit  of  leaf  with  scales  ;  b,  adult 
female ;  c,  wax  scale  under  which  adult  female  lives  ;  d,  larva ;  e,  adult  male. 

food.  It  lays  eggs  under  its  body,  and  thus  also  under  the 
protecting  wax  scale,  and  dies.  From  the  eggs  hatch  active 
little  larval  scale-bugs  with  eyes  and  feelers  and  six  legs. 
They  crawl  from  under  the  wax  scale  and  roam  about  over 
the  orange  tree.  Finally,  they  settle  down,  thrusting  their 
sucking  beak  into  the  plant  tissues,  and  cast  their  skin. 
The  females  lose  at  this  molt  their  legs  and  eyes  and 


PARASITISM  AND  DEGENERATION  197 

feelers.  Each  becomes  a  mere  motionless  sac  capable  only 
of  sucking  up  sap  and  of  laying  eggs.  The  young  males, 
however,  lose  their  sucking  beak  and  can  no  longer  take 
food,  but  they  gain  a  pair  of  wings  and  an  additional  pair 
of  eyes.  They  fly  about  and  fertilize  the  sac-like  females, 
which  then  molt  again  and  secrete  the  thin  wax  scale  over 
them. 

Throughout  the  animal  kingdom  loss  of  the  need  of 
movement  is  followed  by  the  loss  of  the  power  to  move,  and 
of  all  structures  related  to  it. 

103.  Degeneration  through  other  causes. — Loss  of  certain 
organs  may  occur  through  other  causes  than  parasitism  and 
a  fixed  life.  Many  insects  live  but  a  short  time  in  their 
adult  stage.  May-flies  live  for  but  a  few  hours  or,  at  most, 
a  few  days.  They  do  not  need  to  take  food  to  sustain  life 
for  so  short  a  time,  and  so  their  mouth  parts  have  become 
rudimentary  and  f  unctionless  or  are  entirely  lost.  This  is 
true  of  some  moths  and  numerous  other  specially  short- 
lived insects.  Among  the  social  insects  the  workers  of  the 
termites  and  of  the  true  ants  are  wingless,  although  they 
are  born  of  winged  parents,  and  are  descendants  of  winged 
ancestors.  The  modification  of  structure  dependent  upon 
the  division  of  labor  among  the  individuals  of  the  com- 
munity has  taken  the  form,  in  the  case  of  the  workers,  of  a 
degeneration  in  the  loss  of  the  wings.  Insects  that  live 
in  caves  are  mostly  blind ;  they  have  lost  the  eyes,  whose 
function  could  not  be  exercised  in  the  darkness  of  the  cave. 
Certain  island-inhabiting  insects  have  lost  their  wings, 
flight  being  attended  with  too  much  danger.  The  strong 
sea-breezes  may  at  any  time  carry  a  flying  insect  off  the 
small  island  to  sea.  Only  those  which  do  not  fly  much  sur- 
vive, and  by  natural  selection  wingless  .breeds  or  species  are 
produced.  Finally,  we  may  mention  the  great  modifications 
of  structure,  often  resulting  in  the  loss  of  certain  organs, 
which  take  place  to  produce  protective  resemblances  (see 
Chapter  XII).  In  such  cases  the  body  may  be  modified  in 
14 


198  ANIMAL  LIFE 

color  and  shape  so  as  to  resemble  some  part  of  the  envi- 
ronment, and  thus  the  animal  may  be  unperceived  by  its 
enemies.  Many  insects  have  lost  their  wings  through  this 
cause. 

104.  Immediate  causes  of  degeneration. — When  we  say 
that  a  parasitic  or  quiescent  mode  of  life  leads  to  or  causes 
degeneration,  we  have  explained  the  stimulus  or  the  ulti- 
mate  cause   of    degenerative   changes,  but  we  have  not 
shown  just  how  parasitism  or  quiescence  actually  produces 
these  changes.     Degeneration  or  the  atrophy  and  disap- 
pearance of  organs  or  parts  of  a  body  is  often  said  to  be 
due  to  disuse.     That  is,  the  disuse  of  a  part  is  believed  by 
many  naturalists  to  be  the  sufficient  cause  for  its  gradual 
dwindling  and  final  loss.     That  disuse  can  so  affect  parts 
of  a  body  during  the  lifetime  of  an  individual  is  true.     A 
muscle  unused  becomes  soft  and  flabby  and  small.    Whether 
the  effects  of  such  disuse  can  be  inherited,  however,  is  open 
to,  serious  doubt.     Such  inheritance  must  be  assumed  if 
disuse  is  to  account  for  the  gradual  growing  less  and  final 
disappearance  of  an  organ  in  the  course  of  many  genera- 
tions.    Some  naturalists  believe  that  the  results  of  such 
disuse  can  be  inherited,  but  as  yet  such  belief  rests  on  no 
certain  knowledge.     If  characters  assumed  during  the  life- 
time of  the  individual  are  subject  to  inheritance,  disuse 
alone  may  explain  degeneration.    If  not,  some  other  imme- 
diate cause,  or  some  other  cause  along  with  disuse,  must 
be  found.    Such  a  cause  must  be  sought  for  in  the  action  of 
natural  selection,  preserving  the  advantages  of  simplicity  of 
structure  where  action  is  not  required. 

105.  Advantages  and  disadvantages  of  parasitism  and  de- 
generation.— We  are  accustomed,  perhaps,  to  think  of  degen- 
eration as  necessarily  implying  a  disadvantage  in  life.     A 
degenerate  animal  is  considered  to  be  not  the  equal  of  a  non- 
degenerate  animal,  and  this  would  be  true  if  both  kinds  of 
animals  had  to  face  the  same  conditions  of  life.     The  blind, 
footless,  simple,  degenerate  animal  could  not  cope  with  the 


PARASITISM  AND  DEGENERATION  19$ 

active,  keen-sighted,  highly  organized  non-degenerate  in 
free  competition.  But  free  competition  is  exactly  what 
the  degenerate  animal  has  nothing  to  do  with.  Certainly 
the  Sacculina  lives  successfully ;  it  is  well  adapted  for  its 
own  peculiar  kind  of  life.  For  the  life  of  a  scale  insect, 
no  better  type  of  structure  could  be  devised.  A  parasite 
enjoys  certain  obvious  advantages  in  life,  and  even  extreme 
degeneration  is  no  drawback,  but  rather  favors  it  in  the 
advantageousness  of  its  sheltered  and  easy  life.  As  long 
as  tho  host  is  successful  in  eluding  its  enemies  and  avoid- 
ing accident  and  injury,  the  parasite  is  safe.  It  needs  to 
exercise  no  activity  or  vigilance  of  its  own ;  its  life  is  easy 
as  long  as  its  host  lives.  But  the  disadvantages  of  para- 
sitism and  degeneration  are  apparent  also.  The  fate  of  the 
parasite  is  usually  bound  up  with  the  fate  of  the  host. 
When  the  enemy  of  the  host  crab  prevails,  the  Sacculina 
goes  down  without  a  chance  to  struggle  in  its  own  defense. 
But  far  more  important  than  the  disadvantage  in  such  par- 
ticular or  individual  cases  is  the  disadvantage  of  the  fact 
that  the  parasite  can  not  adapt  itself  in  any  considerable 
degree  to  new  conditions.  It  has  become  so  specialized, 
so  greatly  modified  and  changed  to  adapt  itself  to  the  one 
set  of  conditions  under  which  it  now  lives ;  it  has  gone  so- 
far  in  its  giving  up  of  organs  and  body  parts,  that  if  pres- 
ent conditions  should  change  and  new  ones  come  to  exist, 
the  parasite  could  not  adapt  itself  to  them.  The  independ- 
ent, active  animal  with  all  its  organs  and  all  its  functions 
intact,  holds  itself,  one  may  say,  ready  and  able  to  adapt 
itself  to  any  new  conditions  of  life  which  may  gradually 
come  into  existence.  The  parasite  has  risked  everything 
for  the  sake  of  a  sure  and  easy  life  under  the  presently 
existing  conditions.  Change  of  conditions  means  its  ex- 
tinction. 

106.  Human  degeneration. — It  is  not  proposed  in  these 
pages  to  discuss  the  application  of  the  laws  of  animal  life 
to  man.  But  each  and  every  one  extends  upward,  and  can 


200  ANIMAL  LIFE 

be  traced  in  the  relation  of  men  and  society.  Thus,  among 
men  as  among  animals,  self-dependence  favors  complexity 
of  power.  Dependence,  parasitism,  quiescence  favor  de- 
generation. Degeneration  means  loss  of  complexity,  the 
narrowing  of  the  range  of  powers  and  capabilities.  It  is 
not  necessarily  a  phase  of  disease  or  the  precursor  of  death. 
But  as  intellectual  and  moral  excellence  are  matters  associ- 
ated with  high  development  in  man,  dependence  is  unfa- 
vorable to  them. 

Degeneration  has  been  called  animal  pauperism.  Pau- 
perism in  all  its  forms,  whether  due  to  idleness,  pampering, 
or  misery,  is  human  degeneration.  It  has  been  shown  that 
a  large  part  of  the  criminality  and  pauperism  among  men 
is  hereditary,  due  to  the  survival  of  the  tendency  toward 
living  at  the  expense  of  others.  The  tendency  to  live  with- 
out self-activity  passes  from  generation  to  generation. 
Beggary  is  more  profitable  than  unskilled  and  inefficient 
labor,  and  our  ways  of  careless  charity  tend  to  propagate 
the  beggar.  That  form  of  charity  which  does  not  render 
its  recipient  self -helpful  is  an  incentive  toward  degenera- 
tion. Withdrawal  from  the  competition  of  life,  withdrawal 
from  self-helpful  activity,  aided  by  the  voluntary  or  invol- 
untary assistance  of  others — these  factors  bring  about  de- 
generation. The  same  results  follow  in  all  ages  and  with 
all  races,  with  the  lower  animals  as  with  men. 


CHAPTEE  XII 

PROTECTIVE   RESEMBLANCES,   AND    MIMICRY 

107.  Protective  resemblance  defined. — If  a  grasshopper 
be  startled  from  the  ground,  you  may  watch  it  and  deter- 
mine exactly  where  it  alights  after  its  leap  or  flight,  and 
yet,  on  going  to  the  spot,  be  wholly  unable  to  find  it.  The 
colors  and  marking  of  the  insect  so  harmonize  with  its  sur- 
roundings of  soil  and  vegetation  that  it  is  nearly  indistin- 
guishable as  long  as  it  remains  at  rest.  And  if  you  were 
intent  on  capturing  grasshoppers  for  fish-bait,  this  resem- 
blance in  appearance  to  their  surroundings  would  be  very 
annoying  to  you,  while  it  would  be  a  great  advantage  to 
the  grasshoppers,  protecting  some  of  them  from  capture  and 
death.  This  is  protective  resemblance.  Mere  casual  obser- 
vation reveals  to  us  that  such  instances  of  protective  resem- 
blance are  very  common  among  animals.  A  rabbit  or  grouse 
crouching  close  to  the  ground  and  remaining  motionless 
is  almost  indistinguishable.  Green  caterpillars  lying  out- 
stretched along  green  grass-blades  or  on  green  leaves  may 
be  touched  before  being  recognized  by  sight.  In  arctic 
regions  of  perpetual  snow  the  polar  bears,  the  snowy  arctic 
foxes,  and  the  hares  are  all  pure  white  instead  of  brown 
and  red  and  gray  like  their  cousins  of  temperate  and  warm 
regions.  Animals  of  the  desert  are  almost  without  excep- 
tion obscurely  mottled  with  gray  and  sand  color,  so  as  to 
harmonize  with  their  surroundings. 

In  the  struggle  for  existence  anything  that  may  give 
an  animal  an  advantage,  however  slight,  may  be  sufficient 
to  turn  the  scale  in  favor  of  the  organism  possessing  the 

201 


202  ANIMAL  LIFE 

advantage.  Such  an  advantage  may  be  swiftness  of  move- 
ment, or  unusual  strength  or  capacity  to  withstand  unfa- 
vorable meteorological  conditions,  or  the  possession  of  such 
color  and  markings  or  peculiar  shape  as  tend  to  conceal  the 
animal  from  its  enemies  or  from  its  prey.  Eesemblances 
may  serve  the  purpose  of  aggression  as  well  as  protection. 
In  the  case  of  the  polar  bears  and  other  predaceous  ani- 
mals that  show  color  likenesses  to  their  surroundings,  the 
resemblance  can  better  be  called  aggressive  than  protective. 
The  concealment  afforded  by  the  resemblance  allows  them 
to  steal  unperceived  on  their  prey.  This,  of  course,  is  an 
advantage  to  them  as  truly  as  escape  from  enemies  would  be. 

We  have  already  seen  that  by  the  action  of  natural 
selection  and  heredity  those  variations  or  conditions  that 
give  animals  advantages  in  the  struggle  for  life  are  pre- 
served and  emphasized.  And  so  it  has  come  about  that 
advantageous  protective  resemblances  are  very  widespread 
among  animals,  and  assume  in  many  cases  extraordinarily 
striking  and  interesting  forms.  In  fact,  the  explanation 
of  much  of  the  coloring  and  patterning  of  animals  depends 
on  this  principle  of  protective  resemblance. 

Before  considering  further  the  general  conditions  of 
protective  resemblances,  it  will  be  advisable  to  refer  to 
specific  examples  classified  roughly  into  groups  or  special 
kinds  of  advantageous  colorings  and  markings. 

108.  General  protective  or  aggressive  resemblance. — As 
examples  of  general  protective  resemblance — that  is,  a  gen- 
eral color  effect  harmonizing  with  the  usual  surroundings 
and  tending  to  hide  or  render  indistinguishable  the  animal 
— may  be  mentioned  the  hue  of  the  green  parrots  of  the 
evergreen  tropical  forests ;  of  the  green  tree-frogs  and  tree- 
snakes  which  live  habitually  in  the  green  foliage ;  of  the 
mottled  gray  and  tawny  lizards,  birds,  and  small  mam- 
mals of  the  deserts ;  and  of  the  white  hares  and  foxes 
and  snowy  owls  and  ptarmigans  of  the  snow-covered  arc- 
tic regions.  Of  the  same  nature  is  the  slaty  blue  of  the 


204:  ANIMAL   LIFE 

gulls  and  terns,  colored  like  the  sea.  In  the  brooks  most 
fishes  are  dark  olive  or  greenish  above  and  white  below. 
To  the  birds  and  other  enemies  which  look  down  on  them 
from  above  they  are  colored  like  the  bottom.  To  their  fish 
enemies  which  look  up  from  below,  their  color  is  like  the 
white  light  above  them,  and  their  forms  are  not  clearly 
seen.  The  fishes  of  the  deep  sea  in  perpetual  darkness  are 


FIG.  126. — Alligator  lizard  (Gerrhonotus  scincicaudd)  on  granite  rock.    Photograph 
by  J.  O.  SNYDEB,  Stanford  University,  California. 

inky  violet  in  color  below  as  well  as  above.  Those  that 
live  among  sea-weeds  are  red,  grass-green,  or  olive,  like 
the  plants  they  frequent.  General  protective  resemblance 
is  very  widespread  among  animals,  and  is  not  easily  appre- 
ciated when  the  animal  is  seen  in  museums  or  zoological 
gardens — that  is,  away  from  its  natural  or  normal  environ- 
ment. A  modification  of  general  color  resemblance  found 
in  many  animals  may  be  called  variable  protective  resem- 
blance. Certain  hares  and  other  animals  that  live  in 
northern  latitudes  are  wholly  white  during  the  winter  when 
the  snow  covers  everything,  but  in  summer,  when  much  of 
the  snow  melts,  revealing  the  brown  and  gray  rocks  and 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY      205 


withered  leaves,  these  creatures  change  color,  putting  on 
a  grayish  and  brownish  coat  of  hair.  The  ptarmigan  of 
the  Kocky  Mountains  (one  of  the  grouse),  which  lives  on 
the  snow  and  rocks  of  the  high  peaks,  is  almost  wholly 
white  in  winter,  but  in  summer  when  most  of  the  snow  is 
melted  its  plumage  is  chiefly  brown.  On  the  campus  at 
Stanford  University  there  is  a  little  pond  whose  shores  are 
covered  in  some  places  with  bits  of  bluish  rock,  in  other 
places  with  bits  of  reddish  rock,  and  in  still  other  places 
with  sand.  A  small  insect  called  the  toad-bug  ( Oalgulus 
oculatus)  lives  abundantly  on  the  banks  of  this  pond. 
Specimens  collected  from  the  blue  rocks  are  bluish  in 
color,  those  from  the  red  rocks  are  reddish,  and  those  from 
the  sand  are  sand-colored.  Such  changes  of  color  to  suit 
the  changing  surroundings  can  be  quickly  made  in  the  case 
of  some  animals.  The  chameleons  of  the  tropics,  whose 
skin  changes  color  momentarily  from  green  to  brown, 
blackish  or  golden,  is  an  excellent  example  of  this  highly 
specialized  condition.  The  same  change  is  shown  by  a 
small  lizard  of  our  Southern  States  (Anolius),  which  from  its 
habit  is  called  the  Florida 
chameleon.  There  is  a  lit- 
tle fish  (Oligocottus  snyderi) 
which  is  common  in  the  tide 
pools  of  the  bay  of  Monterey, 
in  California,  whose  color 
changes  quickly  to  harmo- 
nize with  the  different  'colors 
of  the  rocks  it  happens  to 
rest  above.  Some  of  the  tree- 
frogs  show  this  variable  col- 
oring. A  very  striking  in- 
stance of  variable  protective 
resemblance  is  shown  by  the 

chrysalids  of  certain  butterflies.     An  eminent  English  nat- 
uralist collected  many  caterpillars  of  a  certain  species  of 


FIG.  127.— Chrypalid  of  swallow-tail  but- 
terfly (Papilio),  harmonizing  with  the 
bark  on  which  it  rests. 


206 


ANIMAL  LIFE 


butterfly,  and  put  them,  just  as  they  were  about  to  change 
into  pupae  or  chrysalids,  into  various  boxes,  lined  with  paper 
of  different  colors.  The  color  of  the  chrysalid  was  found 


FIG.  128. — Chrysalifl  of  butterfly  (lower  left-hand  proipotion  from  stem),  showing  pro- 
tective resemblance.     Pliuiogrupii  Iroiu  IS'ature. 

to  harmonize  very  plainly  with  the  color  of  the  lining  of 
the  box  in  which  the  chrysalid  hung.  It  is  a  familiar  fact 
to  entomologists  that  most  butterfly  chrysalids  resemble  in 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY      207 

color  and  general  external  appearance  the  surface  of  the 
object  on  which  they  rest  (Figs.  127  and  128). 

109.  Special  protective  resemblance. — Far  more  striking 
are  those  cases  of  protective  resemblance  in  which  the  ani- 
mal resembles  in  color  and  shape,  sometimes  in  extraor- 
dinary detail,  some  particular  object  or  part  of  its  usual 
environment.  Certain  parts  of  the  Atlantic  Ocean  are 
covered  with  great  patches  of  sea-weed  called  the  gulf-weed 
(Sargassum),  and  many  kinds  of  animals — fishes  and  other 
creatures — live  upon  and  among  the  algae.  No  one  can 
fail  to  note  the  extraordinary  color  resemblances  which  exist 
between  those  animals  and  the  weed  itself.  The  gulf-weed 
is  of  an  olive-yellow  color,  and  the  crabs  and  shrimps,  a  cer- 
tain flat-worm,  a  certain  mollusk,  and  a  littlef  fish,  all  of 
which  live  among  the  Sargassum^  are  exactly  of  the  same 
shade  of  yellow  as  the  weed,  and  have  small  white  markings 
on  their  bodies  which  are  characteristic  also  of  the  Sargas- 
sum. The  mouse-fish  or  Sargassum  fish  and  the  little  sea- 
horses, often  attached  to  the  gulf -weed,  show  the  same  traits 
of  coloration  (Fig.  129).  In  the  black  rocks  about  Tahiti 
is  found  the  black  nokee  or  lava-fish  (Emmydrichthys  vul- 
canus)  (Fig.  66),  which  corresponds  perfectly  in  color  and 
form  to  a  piece  of  lava.  This  fish  is  also  noteworthy  for 
having  envenomed  spines  in  the  fin  on  its  back.  The 
slender  grass-green  caterpillars  of  many  moths  and  butter- 
flies resemble  very  closely  the  thin  grass-blades  among 
which  they  live.  The  larvae  of  the  geometrid  moths,  called 
inch-worms  or  span-worms,  are  twig-like  in  appearance, 
and  have  the  habit,  when  disturbed,  of  standing  out  stiffly 
from  the  twig  or  branch  upon  which  they  rest,  so  as  to  re- 
semble in  position  as  well  as  in  color  and  markings  a  short 
or  a  broken  twig.  One  of  the  most  striking  resemblances 
of  this  sort  is  shown  by  the  large  geometrid  larva  illus- 
trated in  Fig.  130,  which  was  found  near  Ithaca,  New  York. 
The  body  of  this  caterpillar  has  a  few  small,  irregular  spots 
or  humps,  resembling  very  exactly  the  scars  left  by  fallen 


PIG.  129.— The  mouse-fish  (Pterophryne  histrio)  in  the  Sargassum  or  gulf -weed.  The 
fishes  are  marked  and  colored  so  as  to  be  nearly  indistinguishable  from  the  masses 
of  the  gulf- weed.  In  the  lower  right-hand  corner  of  figure  are  two  sea-horses,  also 
shaped  and  marked  so  as  to  be  concealed. 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY      209 

buds  or  twigs.     These  caterpillars  have  a  special  muscular 
development  to  enable  them  to  hold  themselves  rigidly  for 


FIG.  130.— A  geometrid  larva  on  a  branch.   (The       FIG.  131.— A  walking-stick  insect 
larva  is  the  upper  right-hand  projection  from  (Diapfieronwra  femorata)   on 

the  stem.)  twig. 

long  times  in  this  trying   attitude.     They  also  lack  the 
middle  prop-legs  of  the  body,  common  to  other  lepidopter- 
15 


210 


ANIMAL   LIFE 


ous  larvae,  the  presence  of  which  would  tend  to  destroy  the 
illusion  so  successfully  carried  out  by  them.  The  common 
walking-stick  (Diapheromera)  (Fig.  131),  with  its  wingless, 
greatly  elongate,  dull-colored  body,  is  an  excellent  example 
of  special  protective  resemblance.  It  is  quite  indistinguish- 
able, when  at  rest,  from  the  twigs  to  which  it  is  clinging. 
Another  member  of  the  family  of  insects  to  which  the  walk- 
ing-stick belongs  is  the  famous  green-leaf  insect  (Phyllium) 

(Fig.  132).  It  is  found  in 
South  America  and  is  of  a 
,bright  green  color,  with  broad 
leaf -like  wings  and  body,  with 
markings  which  imitate  the 
leaf  veins,  and  small  irregu- 
lar yellowish  spots  which 
mimic  decaying  or  stained 
or  fungus-covered  spots  in 
the  leaf. 

There  are  many  butter- 
flies that  resemble  dead 
leaves.  All  our  common 
meadow  browns  ( Grapta), 
brown  and  reddish  butter- 
flies with  ragged-edged  wings, 
that  appear  in  the  autumn 


•*<*, 

FIG.  132.— The  green-leaf  insect 
(Phyllium). 


and  flutter  aimlessly  about  ex- 
actly like  the  falling  leaves, 
show  this  resemblance.  But 
most  remarkable  of  all  is  a 

large  butterfly  (Kallima)  (Fig.  133)  of  the  East  Indian 
region.  The  upper  sides  of  the  wings  are  dark,  with 
purplish  and  orange  markings,  not  at  all  resembling  a 
dead  leaf.  But  the  butterflies  when  at  rest  hold  their 
wings  together  over  the  back,  so  that  only  the  under  sides 
of  the  wings  are  exposed.  The  under  sides  of  Kallima9 s 
wings  are  exactly  the  color  of  a  dead  and  dried  leaf,  and 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY      211 

the  wings  are  so  held  that  all  combine  to  mimic  with  ex- 
traordinary fidelity  a  dead  leaf  still  attached  to  the  twig  by 
a  short  pedicle  or  leaf -stalk  imitated  by  a  short  tail  on  the 


FIG.  IS&.—h'aJlima,  the  "  dead-leaf  butterfly." 

hind  wings,  and  showing  midrib,  oblique  veins,  and,  most 
remarkable  of  all,  two  apparent  holes,  like  those  made  in 
leaves  by  insects,  but  in  the  butterfly  imitated  by  two  small 
circular  spots  free  from  scales  and  hence  clear  and  trans- 


212 


ANIMAL  LIFE 


parent.     With  the  head  and  feelers  concealed  beneath  the 
wings,  it  makes  the  resemblance  wonderfully  exact. 

There   are   numerous   instances   of    special    protective 
resemblance  among  spiders.     Many  spiders  (Fig.  134)  that 


FIG.  134.— Spiders  showing  unusual  shapes  and  patterns,  for  purposes  of 
aggressive  resemblance. 

live  habitually  on  tree  trunks  resemble  bits  of  bark  or  small, 
irregular  masses  of  lichen.  A  whole  family  of  spiders, 
which  live  in  flower-cups  lying  in  wait  for  insects,  are  white 
and  pink  and  party-colored,  resembling  the  markings  of  the 
special  flowers  frequented  by  them.  This  is,  of  course,  a 


PIG.  135.— A  pipe-fish  (Phyllopteryx)  resembling  sea-weed,  in  which  it  lives. 

special  resemblance  not  so  much  for  protection  as  for  ag- 
gression ;  the  insects  coming  to  visit  the  flowers  are  unable 
to  distinguish  the  spiders  and  fall  an  easy  prey  to  them. 

110.  Warning  colors  and  terrifying  appearances.— In  the 
cases  of  advantageous  coloring  and  patterning  so  far  dis- 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY     213 

cussed  the  advantage  to  the  animal  lies  in  the  resemblance 
between  the  animals  and  their  surroundings,  in  the  incon- 
spicuousness  and  concealment  afforded  by  the  coloration. 
But  there  is  another  interesting  phase  of  advantageous 
coloration  in  which  the  advantage  derived  is  in  render- 
ing the  animals  as  conspicuous  and  as  readily  recogniz- 
able as  possible.  While  many  animals  are  very  inconspicu- 
ously colored,  or  are  manifestly  colored  so  as  to  resemble 
their  surroundings,  generally  or  specifically,  many  other 
animals  are  very  brightly  and  conspicuously  colored  and 
patterned.  If  we  are  struck  by  the  numerous  cases  of  imi- 
tative coloring  among  insects,  we  must  be  no  less  impressed 
by  the  many  cases  of  bizarre  and  conspicuous  coloration 
among  them. 

Many  animals,  as  we  well  know,  possess  special  and 
effective  weapons  of  defense,  as  the  poison-fangs  of  the 
venomous  snakes  and  the  stings  of  bees  and  wasps.  Other 
animals,  and  with  these  cases  most  of  us  are  not  so  well 
acquainted,  possess  a  means  of  defense,  or  rather  safety,  in 
being  inedible — that  is,  in  possessing  some  acrid  or  ill- 
tasting  substance  in  the  body  which  renders  them  unpala- 
table to  predaceous  animals.  Many  caterpillars  have  been 
found,  by  observation  in  Nature  and  by  experiment,  to  be 
distasteful  to  insectivorous  birds.  Now,  it  is  obvious  that 
it  would  be  a  great  advantage  to  these  caterpillars  if  they 
could  be  readily  recognized  by  birds,  for  a  severe  stroke  by 
a  bird's  bill  is  about  as  fatal  to  a  caterpillar  as  being  wholly 
eaten.  Its  soft,  distended  body  suffers  mortal  hurt  if  cut 
or  bitten  by  the  bird's  beak.  This  advantage  of  being 
readily  recognizable  is  possessed  by  many  if  not  all  ill- 
tasting  caterpillars  by  being  brilliantly  and  conspicuously 
colored  and  marked.  Such  colors  and  markings  are  called 
warning  colors.  They  are  intended  to  inform  birds  of  the 
fact  that  the  caterpillar  displaying  them  is  an  ill-tasting 
insect,  a  caterpillar  to  be  let  alone.  The  conspicuously 
black-and-yellow  banded  larva  (Fig.  43, 1)  of  the  common 
15 


214  ANIMAL  LIFE 

Monarch  butterfly  is  a  good  example  of  the  possession  of 
warning  colors  by  distasteful  caterpillars. 

These  warning  colors  are  possessed  not  only  by  the  ill- 
tasting  caterpillars,  but  by  many  animals  which  have  spe- 
cial means  of  defense.  The  wasps  and  bees,  provided  with 
stings — dangerous  animals  to  trouble — are  almost  all  con- 
spicuously marked  with  yellow  and  black.  The  lady-bird 
beetles  (Fig.  136),  composing  a  whole  family  of  small  beetles 


FIG.  136.— Two  lady-bird  beetles,  conspicuously  colored  and  marked. 

which  are  all  ill-tasting,  are  brightly  and  conspicuously  col- 
ored and  spotted.  The  Gila  monster  (Heloderma),ihe  only 
poisonous  lizard,  differs  from  most  other  lizards  in  being 
strikingly  patterned  with  black  and  brown.  Some  of  the 
venomous  snakes  are  conspicuously  colored,  as  the  coral 
snakes  (Elaps)  or  coralillos  of  the  tropics.  The  naturalist 
Belt,  whose  observations  in  Nicaragua  have  added  much  to 
our  knowledge  of  tropical  animals,  describes  as  follows  an 
interesting  example  of  warning  colors  in  a  species  of  frog : 
'•'  In  the  woods  around  Santo  Domingo  (Nicaragua)  there 
are  many  frogs.  Some  are  green  or  brown  and  imitate 
green  or  dead  leaves,  and  live  among  foliage.  Others  are 
dull  earth-colored,  and  hide  in  holes  or  under  logs.  All 
these  come  out  only  at  night  to  feed,  and  they  are  all 
preyed  upon  by  snakes  and  birds.  In  contrast  with  these 
obscurely  colored  species,  another  little  frog  hops  about  in 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY      215 

the  daytime,  dressed  in  a  bright  livery  of  red  and  blue. 
He  can  not  be  mistaken  for  any  other,  and  his  naming 
breast  and  blue  stockings  show  that  he  does  not  court  con- 
cealment. He  is  very  abundant  in  the  damp  woods,  and  I 
was  convinced  he  was  uneatable  so  soon  as.  I  made  his 
acquaintance  and  saw  the  happy  sense  of  security  with 
which  he  hopped  about.  I  took  a  few  specimens  home 
with  me,  and  tried  my  fowls  and  ducks  with  them,  but 
none  would  touch  them.  At  last,  by  throwing  down  pieces 
of  meat,  for  which  there  was  a  great  competition  among 
them,  I  managed  to  entice  a  young  duck  into  snatching  up 
one  of  the  little  frogs.  Instead  of  swallowing  it,  however, 
it  instantly  threw  it  out  of  its  mouth,  and  went  about  jerk- 
ing its  head,  as  if  trying  to  throw  off  some  unpleasant 
taste." 

Certain  animals  which  are  without  special  means  of 
defense  and  are  not  at  all  formidable  or  dangerous  are  yet 
so  marked  or  shaped  and  so  behave  as  to  present  a  threat- 
ening or  terrifying  appearance.  The  large  green  caterpil- 
lars (Fig.  137)  of  the  Sphinx  moths — the  tomato-worm  is  a 
familiar  one  of  these  larvas — have  a  formidable-looking, 


FIG.  137.— A  "tomato-worm"  larva  of  the  Sphinx  moth,  Phlegettiontius Carolina, 
showing  terrifying  appearance. 

sharp  horn  on  the  back  of  the  next  to  last  body  ring. 
When  disturbed  they  lift  the  hinder  part  of  the  body,  bear- 
ing the  horn,  and  move  it  about  threateningly.  As  a  mat- 
ter of  fact,  the  horn  is  not  at  all  a  weapon  of  defense,  but  is 
quite  harmless.  ^Numerous  insects  when  disturbed  lift 
the  hind  part  of  the  body,  and  by  making  threatening  mo- 


216 


ANIMAL  LIFE 


ances. 


tions  lead  enemies  to  believe  that  they  possess  a  sting. 
The  striking  eye-spots  of  many  insects  are  believed  by  some 
entomologists  to  be  of  the  nature  of  terrifying  appearances. 
The  larva  (Fig.  138)  of  the  Puss  moth  (Cerurd)  has  been 
often  referred  to  as  a  striking  example  of  terrifying  appear- 
When  one  of  these  larvae  is  disturbed,  "  it  retracts 

its  head  into  the 
first  body  ring  in- 
flating the  mar- 
gin, which  is  of  a 
bright  red  color. 
There  are  two  in- 
tensely black  spots 
on  this  margin  in  the 
appropriate  position  for 
eyes,  and  the  whole  ap- 
pearance is  that  of  a  large 
flat  face  extending  to  the 
outer  edge  of  the  red  mar- 
gin. The  effect  is  an  in- 
tensely exaggerated  cari- 
cature of  a  vertebrate 
face,  which,  is  probably 
alarming  to  the  verte- 
brate enemies  of  the  cat- 
erpillar. .  .  .  The  effect  is  also  greatly  strengthened  by  two 
pink  whips  which  are  swiftly  protruded  from  the  prongs 
of  the  fork  in  which  the  body  terminates.  .  .  .  The  end 
of  the  body  is  at  the  same  time  curved  forward  over  the 
back,  so  that  the  pink  filaments  are  brandished  above  the 
head." 

111.  Alluring  coloration. — A  few  animals  show  what  are 
called  alluring  colors — that  is,  they  display  a  color  pattern 
so  arranged  as  to  resemble  or  mimic  a  flower  or  other  lure^ 
and  thus  to  entice  to  them  other  animals,  their  natural  prey. 
This  is  a  special  kind  of  aggressive  resemblance.  A  species 


FIG.  138.— Larva  of  the  Puss  moth  ( Cerurd). 
Upper  figure  shows  the  larva  as  it  appears 
when  undisturbed  ;  lower  figure,  when  dis- 
turbed.—After  POULTON. 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY      217 

of  predatory  insect  called  a  "  praying-horse  "  (allied  to  the 
genus  Mantis),  found  in  India,  has  the  shape  and  color  of 
an  orchid.  Small  insects  are  attracted  and  fall  a  prey  to  it. 
Certain  Brazilian  fly-catching  birds  have  a  brilliantly  colored 
crest  which  can  be  displayed  in  the  shape  of  a  flower-cup. 
The  insects  attracted  by  the  apparent  flower  furnish  the  fly- 
catcher with  food.  An  Asiatic  lizard  is  wholly  colored  like 
the  sand  upon  which  it  lives  except  for  a  peculiar  red  fold 
of  skin  at  each  angle  of  the  mouth.  This  fold  is  arranged 
in  flower-like  shape,  "  exactly  resembling  a  little  red  flower 
which  grows  in  the  sand."  Insects  attracted  by  these 
flowers  find  out  their  mistake  too  late.  In  the  tribe  of 
fishes  called  the  "  anglers  "  or  fishing  frogs  the  front  rays 
of  the  dorsal  fin  are  prolonged  in  shape  of  long,  slender  fila- 
ments, the  foremost  and  longest  of  which  has  a  flattened 
and  divided  extremity  like  the  bait  on  a  hook.  The  fish 
conceals  itself  in  the  mud  or  in  the  cavities  of  a  coral  reef 
and  waves  the  filaments  back  and  forth.  Small  fish  are  at- 
tracted by  the  lure,  mistaking  it  for  worms  writhing  about 
in  the  water  or  among  the  weeds.  As  they  approach  they 
are  ingulfed  in  the  mouth  of  the  angler,  which  in  some  of 
the  species  is  of  enormous  size.  One  of  these  species  is 
known  to  fishermen  as  the  "  all-mouth."  These  fishes 
(LopMus  piscatorius),  which  live  in  the  mud,  are  colored 
like  mud  or  clay.  Other  forms  of  anglers,  living  among 
coral  reefs,  are  brown  and  red  (Antennarius),  their  colora- 
tion imitating  in  minutest  detail  the  markings  and  out- 
growths on  the  reef  itself,  the  lure  itself  imitating  a  worm 
of  the  reef.  In  a  certain  group  of  deep-sea  anglers,  the  sea- 
devils  (Ceratiidce),  certain  species  show  a  still  further  spe- 
cialization of  the  curious  fishing-rod.  In  one  species  ( Co- 
rynolophus  reinhardti)  (Fig.  54),  living  off  the  coast  of 
Greenland  at  a  depth  of  upward  of  a  mile,  the  fishing-rod 
or  first  dorsal  spine  has  a  luminous  bulb  at  its  tip  around 
which  are  fleshy,  worm-like  streamers.  At  the  abyssal 
depths  of  a  mile,  more  or  less,  frequented  by  these  sea- 


218  ANIMAL  LIFE 

devils  ther-e  is  no  light,  the  inky  darkness  being  absolute. 
This  shining  lure  is  therefore  a  most  effective  means  of 
securing  food. 

112.  Mimicry. — Although  the  word  mimicry  could  often 
have  been  used  aptly  in  the  foregoing  account  of  protective 
resemblances,  it  has  been  reserved  for  use  in  connection 
with  a  certain  specific  group  of  cases.  It  has  been  reserved 
to  be  applied  exclusively  to  those  rather  numerous  instances 
where  an  otherwise  defenseless  animal,  one  without  poison- 
fangs  or  sting,  and  without  an  ill-tasting  substance  in  its 
body,  mimics  some  other  specially  defended  or  inedible  ani- 
mal sufficiently  to  be  mistaken  for  it  and  so  to  escape 
attack.  Such  cases  of  protective  resemblance  are  called 
true  mimicry,  and  they  are  especially  to  be  observed  among 
insects. 

In  Fig.  139  are  pictured  three  familiar  American  butter- 
flies. One  of  these,  the  Monarch  butterfly  (Anosia  plexip- 
pus),  is  perhaps  the  most  abundant  and  widespread  butter- 
fly of  our  country.  It  is  a  fact  well  known  to  entomologists 
that  the  Monarch  is  distasteful  to  birds  and  is  let  alone  by 
them.  It  is  a  conspicuous  butterfly,  being  large  and  chiefly 
of  a  red-brown  color.  The  Viceroy  butterfly  (Basilarchia 
archippus),  also  red-brown  and  much  like  the  Monarch,  is 
not,  as  its  appearance  would  seem  to  indicate,  a  very  near 
relative  of  the  Monarch,  belonging  to  the  same  genus,  but 
on  the  contrary  it  belongs  to  the  same  genus  with  the  third 
butterfly  figured,  the  black  and  white  Basilarchia.  All  the 
butterflies  of  the  genus  Basilarchia  are  black  and  white 
except  this  species,  the  Viceroy,  and  one  other.  The  Vice- 
roy is  not  distasteful  to  birds ;  it  is  edible,  but  it  mimics  the 
inedible  Monarch  so  closely  that  the  deception  is  not  de- 
tected by  the  birds,  and  so  it  is  not  molested. 

In  the  tropics  there  have  been  discovered  numerous 
similar  instances  of  mimicry  by  edible  butterflies  of  inedi- 
ble kinds.  The  members  of  two  great  families  of  butterflies 
(Danaidae  and  Heliconidae)  are  distasteful  to  birds,  and  are 


FIG.  139.— The  mimicking  of  the  inedible  Monarch  butterfly  by  the  edible  Viceroy. 
Upper  figure  is  the  Monarch  (Anosia  plexippus) ;  middle  figure  is  the  Viceroy 
(Basilarchia  archippus) ;  lowest  figure  is  another  member  of  the  same  genus 
(Sasilarchia),  to  show  the  usual  color  pattern  of  the  species  of  the  genus. 


220 


ANIMAL  LIFE 


mimicked  by  members  of  the  other  butterfly  families  (espe- 
cially the  Pieridae),  to  which  family  our  common  white 
cabbage-butterfly  belongs,  and  by  the  swallow-tails  (Papi- 
lionidae). 

The  bees  and  wasps  are  protected  by  their  stings.  They 
are  usually  conspicuous,  being  banded  with  yellow  and  black. 
They  are  mimicked  by  numerous  other  insects,  especially 
moths  and  flies,  two  defenseless  kinds  of  insects.  This 
mimicking  of  bees  and  wasps  by  flies  is  very  common,  and 
can  be  observed  readily  at  any  flowering  shrub.  The  flower- 
flies  (Syrphidae),  which,  with  the  bees,  visit  flowers,  can  be 
distinguished  from  the  bees  only  by  sharp  observing.  When 
these  bees  and  flies  can  be  caught  and  examined  in  hand,  it 
will  be  found  that  the  flies  have  but  two  wings  while  the 
bees  have  four. 

A  remarkable  and  interesting  case  of  mimicry  among 
insects  of  different  orders  is  that  of  certain  South  Ameri- 
can tree-hoppers  (of  the  family  Membracidae,  of  the  order 
Hemiptera),  which  mimic  the  famous  leaf -cutting  ant 
(Saula)  of  the  Amazons  (Fig.  140).  These  ants  have  the 
curious  habit  of  cutting  off,  with  their  sharp  jaws,  bits  of 

green  leaves  and  carry- 
ing them  to  their  nests. 
In  carrying  the  bits  of 
leaves  the  ants  hold  them 
vertically  above  their 
heads.  The  leaf-hoppers 
mimic  the  ants  and  their 
burdens  with  remarka- 
ble exactitude  by  having 
the  back  of  the  body  ele- 
vated in  the  form  of  a 

thin,  jagged-edged  ridge  no  thicker  than  a  leaf.  This  part 
of  the  body  is  green  like  the  leaves,  while  the  under  part 
of  the  body  and  the  legs  are  brown  like  the  ants. 

Some  examples  of  mimicry  among  other  animals  than 


FIG.  140.— Tree-hopper  (Membracidse),  which 
mimics  the  leaf-cutting  ant  (Sauba)  of  Bra- 
zil. (Upper  right-hand  insect  is  the  tree- 

.    hopper.) 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY      221 

insects  are  known,  but  not  many.  The  conspicuously 
marked  venomous  coral-snake  or  coralillos  ( Elaps)  is  mim- 
icked by  certain  non-venomous  snakes  called  king-snakes 
(Lampropettis,  Osceola).  The  pattern  of  red  and  black 
bands  surrounding  the  cylindrical  body  is  perfectly  imi- 
tated. But  whether  this  is  true  mimicry  brought  about 
for  purposes  of  protection  may  be  doubted.  Instances 
among  birds  have  been  described,  and  a  single  case  has 
been  recorded  in  the  class  of  mammals.  But  it  is  among 
the  insects  that  the  best  attested  instances  occur.  The 
simple  fact  of  the  close  resemblance  of  two  widely  related 
animals  can  not  be  taken  to  prove  the  existence  of  mimicry. 
Two  animals  may  both  come  to  resemble  some  particular 
part  in  their  common  environment  and  thus  to  resemble 
closely  each  other.  Here  we  have  simply  two  instances 
of  special  protective  resemblance,  and  not  an  instance  of 
mimicry.  The  student  of  zoology  will  do  well  to  watch 
sharply  for  examples  of  protective  resemblance  or  mimicry, 
for  but  few  of  the  instances  that  undoubtedly  exist  are  as 
yet  known. 

113.  Protective  resemblances  and  mimicry  most  common 
among  insects. — The  large  majority  of  the  preceding  exam- 
ples have  been  taken  from  among  the  insects.  This  is 
explained  by  the  fact  that  the  phenomena  of  protective 
resemblances  and  mimicry  have  been  studied  especially 
among  insects;  the  theory  of  mimicry  was  worked  out 
chiefly  from  the  observation  and  study  of,  the  colors  and 
markings  of  insects  and  of  the  economy  of  insect  life. 
Why  protective  resemblances  and  mimicry  among  insects 
have  been  chiefly  studied  is  because  these  conditions  are 
specially  common  among  insects.  The  great  class  Insecta 
includes  more  than  two  thirds  of  all  the  known  living 
species  of  animals.  The  struggle  for  existence  among  the 
insects  is  especially  severe  and  bitter.  All  kinds  of  "  shifts 
for  a  living  "  are  pushed  to  extremes ;  and  as  insect  colors 
and  patterns  are  especially  varied  and  conspicuous,  it  is 


222  ANIMAL  LIFE 

only  to  be  expected  that  this  useful  modification  of  colors 
and  patterns,  that  results  in  the  striking  phenomena  of 
special  protective  resemblances  and  mimicry,  should  be 
specially  widespread  and  pronounced  among  insects.  More- 
over, they  are  mostly  deficient  in  other  means  of  defense, 
and  seem  to  be  the  favorite  food  for  many  different  kinds 
of  animals.  Protective  resemblance  is  their  best  and  most 
widely  adopted  means  of  preserving  life. 

114.  No  volition  in  mimicry. — The  use  of  the  word  mim- 
icry has  been  criticised  because  it  suggests  the  exercise  of 
volition  or  intent  on  the  part  of  the  mimicking  animal. 
The  student  should  not  entertain  this  conception  of  mim- 
icry.    In  the  use  of   "mimicry"  in  connection  with  the 
phenomena  just   described,  the  biologist  ascribes  to  it  a 
technical  meaning,  which  excludes  any  suggestion  of  voli- 
tion or  intent  on  the  part  of  the  mimic.    Just  how  such 
extraordinary  and  perfect  cases  of  mimicry  as  shown  by 
Phyllium  and  Kallima,  have  come  to  exist  is  a  problem 
whose  solution  is  not  agreed  on  by  naturalists,  but  none  of 
them  makes  volition — the  will  or  intent  of  the  animal — any 
part  of  his  proposed  solution.     Each  case  of  mimicry  is  the 
result  of  a  slow  and  gradual  change,  through  a  long  series 
of  ancestors.     The  mimicry  may  indeed  include  the  adop- 
tion of  certain  habits  of  action  which  strengthen  and  make 
more  pronounced  the  deception  of  shape  and  color.     But 
these  habits,  too,  are  the  result  of  a  long  development,  and 
are  instinctive  or  reflex — that  is,  performed  without  the 
exercise  of  volition  or  reason. 

115.  Color;  its  utility  and  beauty. — The  causes  of  color, 
and  the  uses  of  color  in  animals  and  in  plants  are  subjects 
to  which  naturalists  have  paid  and  are  paying  much  atten- 
tion.    The  subject  of  "  protective  resemblances  and  mim- 
icry" is  only  one,  though  one  of  the  most  interesting, 
branches  or  subordinate  subjects  of  the  general  theory  of 
the  uses  of  color.     Other  uses  are  obvious.     Bright  colors 
and  markings  may  serve  for  the  attraction  of  mates ;  thus 


PROTECTIVE  RESEMBLANCES,  AND  MIMICRY      223 

are  explained  by  some  naturalists  the  brilliant  plumage  of 
the  male  birds,  as  in  the  case  of  the  bird-of-paradise  and 
the  pheasants.  Or  they  may  serve  for  recognition  charac- 
ters, enabling  the  individuals  of  a  band  of  animals  readily 
to  recognize  their  companions ;  the  conspicuous  whiteness 
of  the  short  tail  of  the  antelopes  and  cotton-tail  rabbits, 
the  black  tail  of  the  black-tail  deer,  and  the  white  tail- 
feathers  of  the  meadow-lark,  are  explained  by  many  natu- 
ralists on  this  ground.  Eecognition  marks  of  this  type 
are  especially  numerous  among  the  birds,  hardly  a  species 
being  without  one  or  more  of  them,  if  their  meaning  is  cor- 
rectly interpreted.  The  white  color  of  arctic  animals  may 
be  useful  not  alone  in  rendering  them  inconspicuous,  but 
may  serve  also  a  direct  physiological  function  in  preventing 
the  loss  of  heat  from  the  body  by  radiation.  And  the  dark 
colors  of  animals  may  be  of  value  to  them  in  absorbing  heat 
rays  and  thus  helping  them  to  keep  warm.  But  "  by  far 
the  most  widespread  use  of  color  is  to  assist  an  animal  in 
escaping  from  its  enemies  or  in  capturing  its  prey." 

The  colors  of  an  animal  may  indeed  not  be  useful  to 
it  at  all.  Many  color  patterns  exist  on  present-day  birds 
simply  because,  preserved  by  heredity,  they  are  handed 
down  by  their  ancestors,  to  whom,  under  different  condi- 
tions of  life,  they  may  have  been  of  direct  use.  For  the 
most  part,  however,  we  can  look  on  the  varied  colors  and 
the  striking  patterns  exhibited  by  animals  as  being  in  some 
way  or  another  of  real  use  and  value.  We  can  enjoy  the 
exquisite  coloration  of  the  wings  of  a  butterfly  none  the 
less,  however,  because  we  know  that  these  beautiful  colors 
and  their  arrangement  tend  to  preserve  the  life  of  the 
dainty  creature,  and  have  been  produced  by  the  operation 
of  fixed  laws  of  Nature  working  through  the  ages. 


CHAPTER  XIII 

THE   SPECIAL   SENSES 

116.  Importance  of   the  special  senses. — The  means  by 
which  animals  become  acquainted  with  the  outer  world 
are  the   special  senses,  such  as  feeling,  tasting,  smelling, 
hearing,  and  seeing.     The  behavior  of  animals  with  regard 
to  their  surroundings,  with  regard  to  all  the  world  outside 
of  their  own  body,  depends  upon  what  they  learn  of  this 
outer  world  through  the  exercise  of  these  special  senses. 
Habits  are  formed  on  the  basis  of  experience  or  knowledge 
of  the  outer  world  gained  by  the  special  senses,  and  the 
development  of  the  power  to  reason  or  to  have  sense  de- 
pends on  their  pre-existence. 

117.  Difficulty  of  the  study  of  the  special  senses. — We  are 
accustomed  to  think  of  the  organs  of  the  special  senses  as 
extremely  complex  parts  of  the  body,  and  this  is  certainly 
true  in  the  case  of  the  higher  animals.     In  our  own  body 
the  ears  and  eyes  are  organs  of  most  specialized  and  highly 
developed  condition.     But  we  must  not  overlook  the  fact 
that  the  animal  kingdom  is  composed  of  creatures  of  widely 
varying  degrees  of  organization,  and  that  in  any  considera- 
tion of  matters  common  to  all  animals  those  animals  of 
simplest  and  most  lowly  organization  must  be  studied  as 
well  as  those  of  high  development.     The  study  of  the  spe- 
cial senses  presents  two  phases,  namely,  the  study  of  the 
structure  of  the  organs  of  special  sense,  and  the  study  of 
the  physiology  of  special  sense — that  is,  the  functions  of 
these  organs.     It  will  be  recognized  that  in  the  study  of 
how  other  animals  feel  and  taste  and  smell  and  hear  and 


THE  SPECIAL  SENSES  225 

see,  we  shall  have  to  base  all  our  study  on  our  own  experi- 
ence. We  know  of  hearing  and  seeing  only  by  what  we 
know  of  our  own  hearing  and  seeing ;  but  by  examination 
of  the  structure  of  the  hearing  and  seeing  organs  of  cer- 
tain other  animals,  and  by  observation  and  experiments, 
zoologists  are  convinced  that  some  animals  hear  sounds 
that  we  can  not  hear,  and  some  see  colors  that  we  can 
not  see. 

While  that  phase  of  the  study  of  the  special  senses 
which  concerns  their  structure  may  be  quite  successfully 
undertaken,  the  physiological  phase  of  the  study  of  the 
actual  tasting  and  seeing  and  hearing  of  the  lower  animals 
is  a  matter  of  much  difficulty.  The  condition  and  char- 
acter of  the  special  senses  vary  notably  among  different 
animals.  There  may  even  exist  other  special  senses  than 
the  ones  we  possess.  Some  zoologists  believe  that  certain 
marine  animals  possess  a  "  density  or  pressure  sense  " — 
that  is,  a  sense  which  enables  them  to  tell  approximately 
how  deep  in  the  water  they  may  be  at  any  time.  To 
certain  animals  is  ascribed  a  "  temperature  sense,"  and 
some  zoologists  believe  that  what^we  call  the  homing  in- 
stinct of  animals  as  shown  by^  the  homing  pigeons  and 
honey-bees  and  other  animals,  depends  on  their  possession 
of  a  special  sense  which  man  does  not  possess.  Eecent 
experiments,  however,  seem  to  show  that  the  homing  of 
pigeons  depends  on  their  keen  sight.  In  numerous  animals 
there  exist,  besides  the  organs  of  the  five  special  senses 
which  we  possess,  organs  whose  structure  compels  us  to  be- 
lieve them  to  be  organs  of  special  sense,  but  whose  func- 
tion is  wholly  unknown  to  us.  Thus  in  the  study  of  the 
special  senses  we  are  made  to  see  plainly  that  we  can  not 
rely  simply  on  our  knowledge  of  our  own  body  structure 
for  an  understanding  of  the  structure  and  functions  of 
other  animals. 

118.  Special  senses  of  the  simplest  animals. — In  the  Amceba 
(see  Chapter  I),  that  type  of  the  simplest  animals,  with 
16 


226  ANIMAL  LIFE 

one-celled  body,  without  organs,  and  yet  with  its  capacity 
for  performing  the  necessary  life  processes,  there  are  no 
special  senses  except  one  (perhaps  two).  The  Amoeba  can 
feel.  It  possesses  the  tactile  sense.  And  there  are  no 
special  sense  organs  except  one,  which  is  the  whole  of  the 
outer  surface  of  the  body.  If  the  Amoeba  be  touched  with 
a  fine  point  it  feels  the  touch,  for  the  soft  viscous  proto- 
plasm of  its  body  flows  slowly  away  from  the  foreign  ob- 
ject. The  sense  of  feeling  or  touch,  the  tactile  sense,  is 
the  simplest  or  most  primitive  of  the  special  senses,  and 
the  simplest,  most  primitive  organ  of  special  sense  is  the 
outer  surface  or  skin  of  the  body.  Among  those  simple 
animals  that  possess  the  simplest  organs  of  hearing  and 
perceiving  light,  we  shall  find  these  organs  to  be  simply 
specialized  parts  of  the  skin  or  outer  cell  layer  of  the 
body,  and  it  is  a  fact  that  all  the  special  sense  organs  of 
all  animals  are  derived  or  developed  from  the  outer  cell 
layer,  ectoblast,  of  the  embryo.  This  is  true  also  of  the 
whole  nervous  system,  the  brain  and  spinal  cord  of  the 
vertebrates,  and  the  ganglia  and  nerve  commissures  of 
the  invertebrates.  And  while  in  the  higher  animals  the 
nervous  system  lies  underneath  the  surface  of  the  body, 
in  many  of  the  lower,  many-celled  animals  all  the  ganglia 
and  nerves,  all  of  the  nervous  system,  lie  on  the  outer 
surface  of  the  body,  being  simply  a  specialized  part  of 
the  skin. 

119.  The  sense  of  touch. — In  some  of  the  lower,  many- 
celled  animals,  as  among  the  polyps,  there  are  on  the  skin 
certain  sense  cells,  either  isolated  or  in  small  groups,  which 
seem  to  be  stimulated  not  alone  by  the  touching  of  foreign 
substances,  but  also  by  warmth  and  light.  They  are  not 
limited  to  a  single  special  sense.  They  are  the  primitive 
or  generalized  organs  of  special  sense,  and  can  develop  into 
specialized  organs  for  any  one  of  the  special  senses. 

The  simplest  and  most  widespread  of  these  special 
senses  with,  as  a  whole,  the  simplest  organs,  is  the  tactile 


THE  SPECIAL  SENSES 


227 


sense,  or  the  sense  of  touch.  The  special  organs  of  this 
sense  are  usually  simple  hairs  or  papillae  connecting  with  a 
nerve.  These  tactile  hairs  or  papillae  may  be  distributed 
pretty  evenly  over  most  of  the  body,  or  may  be  mainly  con- 
centrated upon  certain  parts  in  crowded  groups.  Many  of 
the  lower  animals  have  projecting  parts,  like  the  feeling 
tentacles  of  many  marine  invertebrates,  or  the  antennas 
(feelers)  of  crabs  and  insects,  which  are  the  special  seat 
of  the  tactile  organs.  Among  the  vertebrates  the  tactile 
organs  are  either  like  those  of  the  invertebrates,  or  are 
little  sac-like  bodies  of  connective  tissue  in  which  the 
end  of  a  nerve  is  curiously  folded  and  convoluted  (Fig. 
141).  These  little  touch  corpuscles  simply  lie  in  the  cell 
layer  of  the  skin,  covered  over  thinly  by  the  cuticle.  Some- 
times they  are  simply  free,  branched 
nerve-endings  in  the  skin.  These 
tactile  corpuscles  or  free  nerve-end- 
ings are  especially  abundant  in  those 
parts  of  the  body  which  can  be  best 
used  for  feeling.  In  man  the  fin- 
ger-tips are  thus  especially  supplied ; 
in  certain  tailed  monkeys  the  tip  of 
the  tail,  and  in  hogs  the  end  of  the 
snout.  The  difference  in  abundance 
of  these  tactile  corpuscles  of  the  skin 
can  be  readily  shown  by  experiment. 
With  a  pair  of  compasses,  whose 
points  have  been  slightly  blunted, 
touch  the  skin  of  the  forearm  of  a 

person  who  has  his  eyes  shut,  with  the  points  about  three 
inches  apart  and  in  the  direction  of  the  length  of  the  arm. 
The  person  touched  will  feel  the  points  as  two.  Eepeat 
the  touching  several  times,  gradually  lessening  the  dis- 
tance between  the  points.  When  the  points  are  not  more 
than  an  inch  to  an  inch  and  a  half  apart,  the  person 
touched  will  feel  but  a  single  touch — that  is,  the  touching 


FIG.  141.— Tactile  papilla  of 
skin  of  man.  »,  nerve.  — 
After  KOELLIKER. 


228  ANIMAL  LIFE 

of  both  points  will  give  the  sensation  of  but  a  single  con- 
tact. Eepeat  the  experiment  on  the  tip  of  the  forefinger, 
and  both  points  will  be  felt  until  the  points  are  only  about 
one  tenth  of  an  inch  apart. 

120.  The  sense  of  taste. — The  sense  of  taste  enables  us  to 
test  in  some  degree  the  chemical  constitution  of  substances 
which  are  taken  into  the  mouth  as  food.  We  discriminate 
by  the  taste  organs  between  good  food  and  bad,  well-tasting 
and  ill-tasting.  These  organs  are,  with  us  and  the  other  air- 
breathing  animals,  located  in  fthe  mouth  or  on  the  mouth 
parts.  They  must  be  located  so  as  to  come  into  contact 
with  the  food,  and  it  is  also  necessary  that  the  food  sub- 
stance to  be  tasted  be  made  liquid.  This  is  accomplished 
by  the  fluids  poured  into  the  mouth  from  the  salivary 
glands.  With  the  lower  aquatic  animals  it  is  not  improb- 
able that  taste  organs  are  situated  on  other  parts  of  the 
body  besides  the  mouth,  and  that  taste  is  used  not  only  to 
test  food  substances,  but  also  to  test  the  chemical  char- 
acter of  the  fluid  medium  in  which  they  live. 

The  taste  organs  are  much  like  the  tactile  organs,  ex- 
cept that  the  special  taste  cell  is  exposed,  so  that  small  par- 
ticles of  the  substance  to  be  tasted  can  come  into  actual 
contact  with  it.  The  nerve -ending  is  usually  in  a  small 
raised  papilla  or  depressed  pit.  In  the  simplest  animals 
there  is  no  special  organ  of  taste,  and  yet  Amoeba  and 
other  Protozoa  show  that  they  appreciate  the  chemical  con- 
stitution of  the  liquid  in  which  they  lie.  They  taste — that 
is,  test  the  chemical  constitution  of  the  substances — by 
means  of  their  undifferentiated  body  surface.  The  taste 
organs  are  not  always  to  be  told  from  the  organs  of  smell. 
Where  an  animal  has  a  certain  special  seat  of  smell,  like 
the  nose  of  the  higher  animals,  then  the  special  sense 
organs  of  the  mouth  can  be  fairly  assumed  to  be  taste 
organs ;  but  where  the  seat  of  both  smell  and  taste  is  in 
the  mouth  or  mouth  parts,  it  is  often  impossible  to  distin- 
guish between  the  two  kinds  of  organs. 


THE  SPECIAL  SENSES  229 

In  mammals  taste  organs  are  situated  on  certain  parts  of 
the  tongue,  and  have  the  form  of  rather  large,  low,  broad 
papillae,-  each  bearing  many  small  taste-buds  (Fig.  142). 
In  fishes  similar  papillae  and  buds  have  been  found  in  vari- 
ous places  on  the  sur- 
face of  the  body,  from 
which  it  is  believed  that 
the  sense  of  taste  in 
fishes  is  not  limited  to 
the  mouth.  In  insects 
the  taste  -  papillae  and 
taste -pits  are  grouped 

in  certain  places  On  the  FIG.  142.-Vertical  section  of  large  papilla  on 
mOUth  parts,  being  eS-  ^  °f  a  Calf ;  «•*•  taste-buds. -After 

pecially    abundant    on 

the  tips  of  small,  segmented,  feeler-like  processes  called 
palpi,  which  project  from  the  under  lip  and  from  the  so- 
called  maxillae. 

121.  The  sense  of  smelL — Smelling  and  tasting  are  closely 
allied,  the  one  testing  substances  dissolved,  the  other  test- 
ing substances  vaporized.  The  organs  of  the  sense  of 
smell  are,  like  those  of  taste,  simple  nerve-endings  in  papil- 
lae or  pits.  The  substance  to  be  smelled  must,  however, 
be  in  a  very  finely  divided  form ;  it  must  come  to  the  or- 
gans of  smell  as  a  gas  or  vapor,  and  not,  as  to  the  organs  of 
taste,  in  liquid  condition.  The  organs  of  smell  are  situated 
usually  on  the  head,  but  as  the  sense  of  smell  is  used  not 
alone  for  the  testing  of  food,  but  for  many  other  purposes, 
the  organs  of  smell  are  not,  like  those  of  taste,  situated 
principally  in  or  near  the  mouth.  Smell  is  a  special  sense 
of  much  wider  range  of  use  than  taste.  By  smell  animals 
can  discover  food,  avoid  enemies,  and  find  their  mates. 
They  can  test  the  air  they  breathe  as  well  as  the  food  they 
eat.  In  the  matter  of  the  testing  of  food  the  senses  of 
both  taste  and  smell  are  constantly  used,  and  are  indeed 
intimately  associated. 
16 


230 


ANIMAL  LIFE 


The  sense  of  smell  varies  a  great  deal  in  its  degree  of 
development  in  various  animals.  With  the  strictly  aquatic 
animals — and  these  include  most  of  the  lower  invertebrates, 
as  the  polyps,  the  star-fishes,  sea-urchins,  and  most  of  the 
worms  and  mollusks — the  sense  of  smell  is  probably  but 
little  developed.  There  is  little  opportunity  for  a  gas  or 
vapor  to  come  to  these  animals,  and  only  as  a  gas  or  vapor 
can  a  substance  be  smelled.  With  these  animals  the  sense 
of  taste  must  take  the  place  of  the  olfactory  sense.  But 
among  the  insects,  mostly  terrestrial  animals,  there  is  an 
extraordinary  development  of  the  sense  of  smell.  It  is  in- 
deed probably  their  principal  special  sense.  Insects  must 
depend  on  smell  far  more  than  on  sight  or  hearing  for 
the  discovery  of  food,  for  becoming 
aware  of  the  presence  of  their  enemies 
and  of  the  proximity  of  their  mates 
and  companions.  The  organs  of 
smell  of  insects  are  situated  princi- 
pally on  the  antennae  or  feelers,  a 
single  pair  of  which  is  borne  on  the 
head  of  every  insect  (Fig.  143).  That 
many  insects  have  an  extraordinarily 
keen  sense  of  smell  has  been  shown 
by  numerous  experiments,  and  is  con- 
stantly proved  by  well-known  habits. 
If  a  small  bit  of  decaying  flesh  be  in- 
closed in  a  box  so  that  it  is  wholly 
FIG.  i43.-Antenna  of  a  leaf-  concealed,  it  will  nevertheless  soon 
eating  beetle,  showing  be  found  by  the  flies  and  carrion 
beetles  that  either  feed  on  carrion 
pr  must  always  lay  their  eggs  in  de- 
caying matter  so  that  their  carrion-eating  larvae  may  be 
provided  with  food.  It  is  believed  that  ants  find  their 
way  back  to  their  nests  by  the  sense  of  smell,  and  that 
they  can  recognize  by  scent  among  hundreds  of  individ- 
uals taken  from  various  communities  the  members  of  their 


smelling-pits   on   the  ex- 
panded terminal  segments. 


THE  SPECIAL  SENSES  231 

own  community.  In  the  insectary  at  Cornell  University, 
a  few  years  ago,  a  few  females  of  the  beautiful  promethea 
moth  (Callosamia  promethea)  were  inclosed  in  a  box, 
which  was  kept  inside  the  insectary  building.  No  males 
had  been  seen  about  the  insectary  nor  in  its  immediate 
vicinity,  although  they  had  been  sought  for  by  collectors. 
A  few  hours  after  the  beginning  of  the  captivity  of  the 
female  moths  there  were  forty  male  prometheas  fluttering 
about  over  the  glass  roof  of  the  insectary.  They  could  not 


FIG.  144.— Promethea  moth,  male,  shovring  specialized  antennae. 

see  the  females,  and  yet  had  discovered  their  presence  in 
the  building.  The  discovery  was  undoubtedly  made  by  the 
sense  of  smell.  These  moths  have  very  elaborately  devel- 
oped antennae  (Fig.  144),  finely  branched  or  feathered, 
affording  opportunity  for  the  existence  of  very  many  smell- 
ing-pits. 

The  keenness  of  scent  of  hounds  and  bird  dogs  is  famil- 
iar to  all,  although  ever  a  fresh  source  of  astonishment  as 
we  watch  these  animals  when  hunting.  We  recently 
watched  a  retriever  dog  select  unerringly,  by  the  sense  of 
smell,  any  particular  duck  out  of  a  pile  of  a  hundred.  In 


232  ANIMAL  LIFE 

the  case  of  man  the  sense  of  smell  is  not  nearly  so  well 
developed  as  among  many  of  the  other  vertebrates.  This 
inferiority  is  largely  due  to  degeneration  through  lessened 
need;  for  in  Indians  and  primitive  races  the  sense  of 
smell  is  keener  and  better  developed  than  in  civilized 
races.  Where  man  has  to  make  his  living  by  hunting,  and 
has  to  avoid  his  enemies  of  jungle  and  plain,  his  special 
senses  are  better  developed  than  where  the  necessity  of 
protection  and  advantage  by  means  of  such  keenness  of 
scent  and  hearing  is  done  away  with  by  the  arts  of  civi- 
lization. 

122.  The  sense  of  hearing. — Hearing  is  the  perception 
of  certain  vibrations  of  bodies.  These  vibrations  give  rise 
to  waves — sound  waves  as  they  are  called-— which  proceed 
from  the  vibrating  body  in  all  directions,  and  which,  com- 
ing to  an  animal,  stimulate  the  special  auditory  or  hearing 
organs,  that  transmit  this  stimulation  along  the  auditory 
nerve  to  the  brain,  where  it  is  translated  as  sound.  These 
sound  waves  come  to  animals  usually  through  the  air,  or, 
in  the  case  of  aquatic  animals,  through  water,  or  through 
both  air  and  water. 

The  organs  of  hearing  are  of  very  complex  structure 
in  the  case  of  man  and  the  higher  vertebrates.  Our  ears, 
which  are  adapted  for  perceiving  or  being  stimulated  by 
vibrations  ranging  from  16  to  40,000  a  second — that  is,  for 
hearing  all  those  sounds  produced  by  vibrations  of  a  rapid- 
ity not  less  than  16  to  a  second  nor  greater  than  40,000  to 
a  second — are  of  such  complexity  of  structure  that  many 
pages  would  be  required  for  their  description.  But  among 
the  lower  or  less  highly  organized  animals  the  ears,  or  au- 
ditory organs,  are  much  simpler. 

In  most  animals  the  auditory  organs  show  the  common 
characteristic  of  being  wholly  composed  of,  or  having  as 
an  essential  part,  a  small  sac  filled  with  liquid  in  which 
one  or  more  tiny  spherical  hard  bodies  called  otoliths  are 
held.  This  auditory  sac  is  formed  of  or  lined  internally  by 


THE  SPECIAL  SENSES 


233 


auditory  cells,  specialized  nerve  cells,  which  often  bear 
delicate  vibratile  hairs  (Fig.  145).  Auditory  organs  of  this 
general  character  are  known  among  the  polyps,  the  worms, 
the  crustaceans,  and  the  mollusks.  In  the  common  cray- 
fish the  "  ears  "  are  situated  in  the  basal  segment  of  the 
inner  antennae  or  feelers  (Fig.  146).  They  consist  each  of 
a  small  sac  filled  with  liquid  in  which 
are  suspended  several  grains  of  sand 
or  other  hard  bodies.  The  inner 


FIG.  145.— Auditory  organ  of  a  mollusk.  a,  audi- 
tory nerve ;  b,  outer  wall  of  connective  tissue ; 
c,  cells  with  auditory  hairs  ;  d,  otolith.— After 
LETDIG. 


FIG.  146.  —  Antenna  of 
cray -fish,  with  audi- 
tory sac  at  base. — 
After  HUXLEY. 


surface  of  the  sac  is  lined  with  fine  auditory  hairs.  The 
sound  waves  coming  through  the  air  or  water  outside  strike 
against  this  sac,  which  lies  in  a  hollow  on  the  upper  or 
outer  side  of  the  antennae.  The  sound  waves  are  taken  up 
by  the  contents  of  the  sac  and  stimulate  the  fine  hairs, 
which  in  turn  give  this  stimulus  to  the  nerves  which  run 
from  them  to  the  principal  auditory  nerve  and  thus  to  the 
brain  of  the  cray-fish.  Among  the  insects  other  kinds  of 
auditory  organs  exist.  The  common  locust  or  grasshopper 


234 


ANIMAL  LIFE 


has  on  the  upper  surface  of  the  first  abdominal  segment 
a  pair  of  tympana  or  ear-drums  (Fig.  147),  composed  sim- 
ply of  the  thinned,  tightly  stretched  chitinous 
cuticle  of  the  body.  On  the  inner  surface  of  this 


FIG.  147.— Grasshopper,  showing  auditory  organ  (a.  o.)  in  first  segment  of  abdomen. 
(Wings  of  one  side  removed.) 

ear-drum  there  are  a  tiny  auditory  sac,  a  fine  nerve  lead- 
ing from  it  to  a  small  auditory  ganglion  lying  near  the 
tympanum,  and  a  large  nerve  leading  from  this  ganglion 
to  one  of  the  larger  ganglia  situated  on  the  floor  of  the 


a.o 

Pis.  148.— A  cricket,  showing  auditory  organ  (a.  o.)  in  fore-leg. 

thorax.  In  the  crickets  and  katydids,  insects  related  to 
the  locusts,  the  auditory  organs  or  ears  are  situated  in  the 
fore-legs  (Fig.  148). 

Certain  other  insects,  as  the  mosquitoes  and  other  midges 


THE  SPECIAL  SENSES 


235 


or  gnats,  undoubtedly  hear  by  means  of  numerous  delicate 

hairs  borne  on  the  antennae.     The  male  mosquitoes  (Fig. 

149)  have  many  hundreds  of  these  long,  fine  antennal  hairs, 

and  on  the  sounding  of  a  tuning-fork  these  hairs  have  been 

observed  to  vibrate  strongly.     In  the  base  of  each  antenna 

there  is  a  most  elaborate  organ, 

composed    of    fine     chitinous 

rods,  and  accompanying  nerves 

and  nerve  cells  whose  function 

it  is  to  take  up  and  transmit 

through  the  auditory  nerve  to 

the  brain  the  stimuli  received 

from  the  external  auditory 

hairs. 

123.  Sound -making.  —  The 
sense  of  hearing  enables  ani- 
mals not  only  to  hear  the 
warning  natural  sounds  of 
storms  and  falling  trees  and 
plunging  avalanches,  but  the 
sounds  made  by  each  other. 
Sound-making  among  animals 
serves  to  aid  in  frightening 
away  enemies  or  in  warning 
companions  of  their  approach, 
for  recognition  among  mates 

and  members  of  a  band  or  species,  for  the  attracting  and 
wooing  of  mates,  and  for  the  interchange  of  information. 
With  the  cries  and  roars  of  mammals,  the  songs  of  birds, 
and  the  shrilling  and  calling  of  insects  all  of  us  are  familiar. 
These  are  all  sounds  that  can  be  heard  by  the  human  ear. 
But  that  there  are  many  sounds  made  by  animals  that 
we  can  not  hear — that  is,  that  are  of  too  high  a  pitch  for 
our  hearing  organs  to  be  stimulated  by — is  believed  by  nat- 
uralists. Especially  is  this  almost  certainly  true  in  the  case 
of  the  insects.  The  peculiar  sound-producing  organs  of 


FIG.  149.— A  male  mosquito,  showing 
auditory  hairs  (a.  h.)  on  the  an- 
tennae. 


236  ANIMAL  LIFE 

many  sound-making  insects  are  known  ;  but  certain  other 
insects,  which  make  no  sound  that  we  can  hear,  neverthe- 
less possess  similar  sound-making  organs. 

Sound  is  produced  by  mammals  and  birds  by  the  strik- 
ing of  the  air  which  goes  to  and  comes  from  the  lungs 
against  certain  vibratory  cords  or  flaps  in  the  air-tubes. 
Sounds  made  by  this  vibration  are  re-enforced  and  made 
louder  by  arrangements  of  the  air-tubes  and  mouth  for 
resonance,  and  the  character  or  quality  of  the  sound  is 
modified  at  will  to  a  greater  or  less  degree  by  the  lips  and 
teeth  and  other  mouth  structures.  Sounds  so  made  are 
said  to  be  produced  by  a  voice,  or  animals  making  sounds 
in  this  way  are  said  to  possess  a  voice.  Animals  possessing 
a  voice  have  far  more  range  and  variety  in  their  sound- 
making  than  most  of  the  animals  which  produce  sounds  in 
other  ways.  The  marvelous  variety  and  th6  great  strength 
of  the  singing  of  birds  and  of  the  cries  and  roars  of  mam- 
mals are  unequaled  by  the  sounds  of  any  other  animals. 

But  many  animals  without  a  voice — that  is,  which  do  not 
make  sounds  from  the  air-tubes — make  sounds,  and  some 
of  them,  as  certain  insects,  show  much  variety  and  range 
in  their  singing.  The  sounds  of  insects  are  made  by  the 
rapid  vibrations  of  the  wings,  as  the  humming  or  buzzing 
of  bees  and  flies,  by  the  passage  of  air  out  or  into  the  body 
through  the  many  breathing  pores  or  spiracles  (a  kind 
of  voice),  by  the  vibration  of  a  stretched  membrane  or 
tympanum,  as  the  loud  shrilling  of  the  cicada,  and  most 
commonly  by  stridulation — that  is,  by  rubbing  together 
two  roughened  parts  of  the  body.  The  male  crickets  and 
the  male  katydids  rub  together  the  bases  of  their  wing 
covers  to  produce  their  shrill  singing.  The  locusts  or 
grasshoppers  make  sounds  when  at  rest  by  rubbing  the 
roughened  inside  of  their  great  leaping  legs  against  the 
upper  surface  of  their  wing  covers,  and  when  in  flight  by 
striking  the  two  wings  of  each  side  together.  Numerous 
other  insects  make  sounds  by  stridulation,  but  many  of 


THE  SPECIAL  SENSES  237 

these  sounds  are  so  feeble  or  so  high  in  pitch  that  they  are 
rarely  heard  by  us.  Certain  butterflies  make  an  odd  click- 
ing sound,  as  do  some  of  the  water-beetles.  In  Japan, 
where  small  things  which  are  beautiful  are  prized  not  less 
than  large  ones,  singing  insects  are  kept  in  cages  and 
highly  valued,  so  that  their  capture  becomes  a  lucrative 
industry,  just  as  it  is  with  song  birds  in  Europe  and  Amer- 
ica. Among  the  many  species  of  Japanese  singing  insects 
is  a  night  cricket,  known  as  the  bridle-bit  insect,  because 
its  note  resembles  the  jingling  of  a  bridle-bit. 

124.  The  sense  of  sight. — Kot  all  animals  have  eyes. 
The  moles  which  live  underground,  insects,  and  other  ani- 
mals that  live  in  caves,  and  the  deep-sea  fishes  which  live 
in  waters  so  deep  that  the  light  of  the  sun  never  comes 
to  them,  have  no  eyes  at  all,  or  have  eyes  of  so  rudimentary 
a  character  that  they  can  no  longer  be  used  for  seeing. 
But  all  these  eyeless  animals  have  no  eyes  because  they 
live  under  conditions  where  eyes  are  useless.  They  have 
lost  their  eyes  by  degeneration.  There  are,  however,  many 
animals  that  have  no  eyes,  nor  have  they  or  their  ancestors 
ever  had  eyes.  These  are  the  simplest,  most  lowly  organ- 
ized animals.  Many,  perhaps  all  eyeless  animals  are,  how- 
ever, capable  of  distinguishing  light  from  darkness.  They 
are  sensitive  to  light.  An  investigator  placed  several  indi- 
viduals of  the  common,  tiny  fresh- water  polyp  (Hydra)  in  a 
glass  cylinder  the  walls  of  which  were  painted  black.  He 
left  a  small  part  of  the  cylinder  unpainted,  and  in  this  part 
of  the  cylinder  where  the  light  penetrated  the  Hydras  all 
gathered.  The  eyeless  maggots  or  larvae  of  flies,  when 
placed  in  the  light  will  wriggle  and  squirm  away  into  dark 
crevices.  They  are  conscious  of  light  when  exposed  to  it, 
and  endeavor  to  shun  it.  Most  plants  turn  their  leaves 
toward  the  light ;  the  sunflowers  turn  on  their  stems  to 
face  the  sun.  Light  seems  to  stimulate  organisms  whether 
they  have  eyes  or  not,  and  the  organisms  either  try  to  get 
into  the  light  or  to  avoid  it.  But  this  is  not  seeing. 


238 


ANIMAL  LIFE 


The  simplest  eyes,  if  we  may  call  them  eyes,  are  not 
capable  of  forming  an  image  or  picture  of  external  objects. 
They  only  make  the  animal  better  capable  of  distinguish- 
ing between  light  and  darkness  or  shadow.  Many  lowly 
organized  animals,  as  some  polyps,  and  worms,  have  certain 
cells  of  the  skin  specially  provided  with  pigment.  These 
cells  grouped  together  form  what  is  called  a  pigment  fleck, 
which  can,  because  of  the  presence  of  the  pigment,  absorb 
more  light  than  the  skin  cells,  and  are  more  sensitive  to 
the  light.  By  such  pigment-flecks,  or  eye-spots,  the  animal 
can  detect,  by  their  shadows,  the  passing  near  them  of  mov- 
ing bodies,  and  thus  be  in  some  measure  informed  of  the 
approach  of  enemies  or  of  prey.  Some  of  these  eye-flecks 
are  provided,  not  simply  with  pigment,  but  with  a  simple 
sort  of  lens  that  serves  to  concentrate  rays  of  light  and 

make  this  simplest 
sort  of  eye  even 
more  sensitive  to 
changes  in  the  in- 
tensity of  light 
(Fig.  150). 

Most  of  the 
many  -  celled  ani- 
mals possess  eyes 
by  means  of  which 
a  picture  of  exter- 
nal objects  more  or  less  nearly  complete  and  perfect  can 
be  formed.  There  is  great  variety  in  the  finer  structure 
of  these  picture-forming  eyes,  but  each  consists  essentially 
of  an  inner  delicate  or  sensitive  nervous  surface  called  the 
retina,  which  is  stimulated  by  light,  and  is  connected  with 
the  brain  by  a  large  optic  nerve,  and  of  a  transparent  light- 
refracting  lens  lying  outside  of  the  retina  and  exposed  to 
the  light.  These  are  the  constant  essential  parts  of  an 
image  -  forming  and  image -perceiving  eye.  In  most  eyes 
there  are  other  accessory  parts  which  may  make  the  whole 


FIG.  150.— The  simple  eye  of  a  jelly-fish  (Lizzia 
koeUikeri).—Miet  O.  and  R.  HERTWIO. 


THE  SPECIAL  SENSES 


239 


eye  an  organ  of  excessively  complicated  structure  and  of 
remarkably  perfect  seeing  capacity.  Our  own  eyes  are 
organs  of  extreme  structural  complexity  and  of  high  de- 
velopment, although  some  of  the  other  vertebrates  have 
undoubtedly  a  keener  and  more  nearly  perfected  sight. 

The  crustaceans  and  insects  have  eyes  of  a  peculiar 
character  called  compound  eyes.  In  addition  most  insects 
have  smaller  simple  eyes.  Each  of  the  compound  eyes  is 
composed  of  many  (from  a  few,  as  in  certain  ants,  to  as 
many  as  twenty-five  thousand,  as  in  certain  beetles)  eye  ele- 
ments, each  eye  element  seeing  independently  of  the  other 
eye  elements  and  seeing  only  a  very  small  part  of  any  ob- 
ject in  front  of  the  whole  eye.  All  these  small  parts  of 
the  external  object  seen  by  the  many  distinct  eye  elements 
are  combined  so  as  to  form  an  image  in  mosaic — that  is, 
made  up  of  separate  small  parts — of  the  external  object. 
If  the  head  of  a  dragon-fly  be  exam- 
ined, it  will  be  seen  that 
two  thirds  or  more  of  the 


FIG.  151.— A  dragon-fly,  showing  the  large  com- 
pound eyes  on  the  head. 


FIG.  152. — Some  of  the  facets 
of  the  compound  eye  of  a 
dragon-fly. 


whole  head  is  made  up  of  the  two  large  compound  eyes 
(Fig.  151),  and  with  a  lens  it  may  be  seen  that  the  outer 
surface  of  each  of  these  eyes  is  composed  of  many  small 
spaces  or  facets  (Fig.  152)  which  are  the  outer  lenses  of 
the  many  eye  elements  composing  the  whole  eye. 


CHAPTEE  XIV 

INSTINCT   AND   REASON 

•  125.  Irritability. — All  animals  of  whatever  degree  of 
organization  show  in  life  the  quality  of  irritability  or  re- 
sponse to  external  stimulus.  Contact  with  external  things 
produces  some  effect  on  each  of  them,  and  this  effect  is 
something  more  than  the  mere  mechanical  effect  on  the 
matter  of  which  the  animal  is  composed.  In  the  one- 
celled  animals  the  functions  of  response  to  external  stimu- 
lus are  not  localized.  They  are  the  property  of  any  part  of 
the  protoplasm  of  the  body.  Just  as  breathing  or  digestion 
is  a  function  of  the  whole  cell,  so  are  sensation  and  response 
in  action.  In  the  higher  or  many-celled  animals  each  of 
these  functions  is  specialized  and  localized.  A  certain  set 
of  cells  is  set  apart  for  each  function,  and  each  organ  or 
series  of  cells  is  released  from  all  functions  save  its  own. 

126.  Nerve  cells  and  fibers. — In  the  development  of  the 
individual  animal  certain  cells  from  the  primitive  external 
layer  or  ectoblast  of  the  embryo  are  set  apart  to  preside 
over  the  relations  of  the  creature  to  its  environment. 
These  cells  are  highly  specialized,  and  while  some  of  them 
are  highly  sensitive,  others  are  adapted  for  carrying  or 
transmitting  the  stimuli  received  by  the  sensitive  cells,  and 
still  others  have  the  function  of  receiving  sense-impressions 
and  of  translating  them  into  impulses  of  motion.  The 
nerve  cells  are  receivers  of  impressions.  These  are  gathered 
together  in  nerve  masses  or  ganglia,  the  largest  of  these 
being  known  as  the  brain, .the  ganglia  in  general  being 
known  as  nerve  centers.  The  nerves  are  of  two  classes. 
240 


INSTINCT  AND  REASON  241 

The  one  class,  called  sensory  nerves,  extends  from  the  skin 
or  other  organ  of  sensation  to  the  nerve  center.  The  nerves 
of  the  other  class,  motor  nerves,  carry  impulses  to  motion. 

127.  The  brain  or  sensorium. — The  brain  or  other  nerve 
center  sits  in  darkness  surrounded  by  a  bony  protecting 
box.     To  this  main  nerve  center,  or  sensorium,  come  the 
nerves  from  all  parts  of  the  body  that  have  sensation, 
the  external  skin  as  well  as  the  special  organs  of  sight, 
hearing,  taste,  smell.     "With  these  come  nerves  bearing  sen- 
sations of  pain,  temperature,  muscular  effort — all  kinds  of 
sensation  which  the  brain  can  receive.     These  nerves  are 
the  sole  sources  of  knowledge  to   any  animal   organism. 
Whatever  idea  its  brain  may  contain  must  be  built  up 
through  these  nerve  impressions.     The  aggregate  of  these 
impressions  constitute  the  world  as  the  organism  knows  it. 
All  sensation  is  related  to  action.     If  an  organism  is  not 
to  act,  it  can  not  feel,  and  the  intensity  of  its  feeling  is 
related  to  its  power  to  act. 

128.  Reflex  action. — These  impressions  brought  to  the 
brain  by  the  sensory  nerves  represent  in  some  degree  the 
facts  in  the  animal's  environment.     They  teach  something 
as  to  its  food  or  its  safety.     The  power  of  locomotion  is 
characteristic  of  animals.    If  they  move,  their  actions  must 
depend  on  the  indications  carried  to  the  nerve  center  from 
the  outside;  if  they  feed  on  living  organisms,  they  must 
seek  their  food ;  if ,  as  in  many  cases,  other  living  organ- 
isms prey  on  them,  they  must  bestir  themselves  to  escape. 
The  impulse  of  hunger  on  the  one  hand  and  of  fear  on  the 
other  are  elemental.     The  sensorium  receives  an  impression 
that  food  exists  in  a  certain  direction.     At  once  an  impulse 
to  motion  is  sent  out  from  it  to  the  muscles  necessary  to 
move  the  body  in  that  direction.     In  the  higher  animals 
these  movements  are  more  rapid  and  more  exact.     This  is 
because  organs  of  sense,  muscles,  nerve  fibers,  and  nerve 
cells  are  all  alike  highly  specialized.     In  the  star-fish  the 
sensation  is  slow,  the  muscular  response  sluggish,  but  the 

17 


ANIMAL  LIFE 

method  remains  the  same.  \This  is  simple  reflex  action,  an 
impulse  from  the  environment  carried  to  the  brain  and 
ihen  unconsciously  reflected  back  as  motion.\  The  impulse 
of  fear  is  of  the  same  nature.  Strike  at  a  dog  with  a  whip, 
:and  he  will  instinctively  shrink  away,  perhaps  with  a  cry. 
Perhaps  he  will  leap  at  you,  and  you  unconsciously  will  try 
to  escape  from  him.  Keflex  action  is  in  general  uncon- 
scious, but  with  animals  as  with  man  it  shades  by  degrees 
into  conscious  action,  and  into  volition  or  action  "  done  on 
purpose." 

129.  Instinct. — Different  one-celled  animals  show  differ- 
•ences  in  method  or  degree  of  response  to  external  influences. 
The  feelers  of  the  Amoeba  will  avoid  contact  with  the  feel- 
ers or  pseudopodia  of  another  Amo&ba,  while  it  does  not 
shrink  from  contact  with  itself  or  with  an  organism  of  un- 
like kind  on  which  it  may  feed.  Most  Protozoa  will  discard 
grains  of  sand,  crystals  of  acid,  or  other  indigestible  object. 
Such  peculiarities  of  different  forms  of  life  constitute  the 
basis  of  instinct. 

V  Instinct  is  automatic  obedience  to  the  demands  of  ex- 
ternal conditions.\  As  these  conditions  vary  with  each  kind 
-of  animal,  so  must  the  demands  vary,  and  from  this  arises 
the  great  variety  actually  seen  in  the  instincts  of  different 
animals.  ^As  the  demands  of  life  become  complex,  so  may 
the  instincts  become  so.  \  The  greater  the  stress  of  envi- 
ronment, the  more  perfect  the  automatism,  for  impulses  to 
safe  action  are  necessarily  adequate  to  the  duty  they  have 
to  perform.  If  the  instinct  were  inadequate,  the  species 
would  have  become  extinct.  The  fact  that  its  individuals 
persist  shows  that  they  are  provided  with  the  instincts 
necessary  to  that  end.  Instinct  differs  from  other  allied 
forms  of  response  to  external  condition  in  being  hereditary, 
continuous  from  generation  to  generation.  This  suffi- 
ciently distinguishes  it  from  reason,  but  the  line  between 
instinct  and  reason  and  other  forms  of  reflex  action  can 
not  be  sharply 


INSTINCT  AND  REASON  243 

It  is  not  necessary  to  consider  here  the  question  of  the 
origin  tff  instincts.  Some  writers  regard  them  as  "  inherited 
habits,"  while  others,  with  apparent  justice,  doubt  if  mere 
habits  or  voluntary  actions  repeated  till  they  become  a 
"  second  nature  "  ever  leave  a  trace  upon  heredity.  Such 
investigators  regard  instinct  as  the  natural  survival  of  those 
methods  of  automatic  response  which  were  most  useful  to 
the  life  of  the  animal,  the  individuals  having  less  effective 
methods  of  reflex  action  having  perished,  leaving  no  pos- 
terity. 

An  example  in  point  would  be  the  homing  instinct  of 
the  fur-seal.  When  the  arctic  winter  descends  on  its  home 
in  the  Pribilof  Islands  in  Bering  Sea,  these  animals  take 
to  the  open  'ocean,  many  of  them  swimming  southward  as 
far  as  the  Santa  Barbara  Islands  in  California,  more  than 
three  thousand  miles  from  home.  While  on  the  long  swim 
they  never  go  on  shore,  but  in  the  spring  they  return  to 
the  northward,  finding  the  little  islands  hidden  in  the  arc- 
tic fogs,  often  landing  on  the  very  spot  from  which  they 
were  driven  by  the  ice  six  months  before,  and  their  arrival 
timed  from  year  to  year  almost  to  the  same  day.  The  per- 
fection of  this  homing  instinct  is  vital  to  their  life.  If 
defective  in  any  individual,  he  would  be  lost  to  the  herd 
and  would  leave  no  descendants.  Those  who  return  be- 
come the  parents  of  the  herd.  As  to  the  others  the  rough 
sea  tells  -no  tales.  We  know  that,  of  those  that  set  forth,  a 
large  percentage  never  comej  back.  To  those  that  return 
the  homing  instinct  has  proved  adequate.  This  must  be  so 
so  long  as  the  race  exists.  jThe  failure  of  instinct  would 
mean  the  extinction  of  the  species.  \ 

130.  Classification  of  instincts. — The  instincts  of  animals 
may  be  roughly  classified  as  to  their  relation  to  the  indi- 
vidual into  egoistic  and  altruistic  instincts. 

Egoistic  instincts  are  those  which  concern  chiefly  the 
individual  animal  itself.  To  this  class  belong  the  instincts 
of  feeding,  those  of  self-defense  and  of  strife,  the  instincts 


244  ANIMAL  LIFE 

of  play,  the  climatic  instincts,  and  environmental  instincts, 
those  which  direct  the  animal's  mode  of  life. 

Altruistic  instincts  are  those  which  relate  to  parent- 
hood and  those  which  are  concerned  with  the  mass  of  indi- 
viduals of  the  same  species.  The  latter  may  be  called  the 
social  instincts.  In  the  former  class,  the  instincts  of  par- 
enthood, may  be  included  the  instincts  of  courtship,  re- 
production, home-making,  nest-building,  and  care  for  the 
young. 

131.  Feeding. — The  instincts  of  feeding  are  primitively 
simple,  growing  complex  through  complex  conditions. 
The  protozoan  absorbs  smaller  creatures  which  contain 
nutriment.  The  sea-anemone  closes  its  tentacles  over  its 
prey.  The  barnacle  waves  its  feelers  to  bring  edible  crea- 
tures within  its  mouth.  The  fish  seizes  its  prey  by  direct 
motion.  The  higher  vertebrates  in  general  do  the  same, 
but  the  conditions  of  life  modify  this  simple  action  to  a 
very  great  degree. 

In  general,  animals  decide  by  reflex  actions  what  is 
suitable  food,  and  by  the  same  processes  they  reject  poisons 
or  unsuitable  substances.  The  dog  rejects  an  apple,  while 
the  horse  rejects  a  piece  of  meat.  Either  will  turn  away 
from  an  offered  stone.  Almost  all  animals  reject  poisons 
instantly.  Those  who  fail  in  this  regard '  in  a  state  of 
nature  die  and  leave  no  descendants.  The  wild  vetches  or 
"  loco-weeds  "  of  the  arid  regions  affect  the  nerve  centers  of 
animals  and  cause  dizziness  or  death.  The  native  ponies 
reject  these  instinctively.  This  may  be  because  all  ponies 
which  have  not  this  reflex  dislike  have  been  destroyed. 
The  imported  horse  has  no  such  instinct  and  is  poisoned. 
Very  few  animals  will  eat  any  poisonous  object  with  which 
their  instincts  are  familiar,  unless  it  be  concealed  from  smell 
and  taste. 

In  some  cases,^very  elaborate  instincts  arise  in  connec- 
tion with  feeding  habits.  \With  the  California  woodpeckers 
(Melanerpes  formicivorus  lairdii)  a  large  number  of  them 


INSTINCT  AND  REASON  245 

together  select  a  live-oak  tree  for  their  operations.  They 
first  bore  its  bark  full  of  holes,  each  large  enough  to  hold 
an  acorn.  Then  into  each  hole  an  acorn  is  thrust  (Figs. 
61  and  62).  Only  one  tree  in  several  square  miles  may  be 
selected,  and  when  their  work  is  finished  all  those  inter- 
ested go  about  their  business  elsewhere.  At  irregular  in- 
tervals a  dozen  or  so  come  back  with  much  clamorous  dis- 
cussion to  look  at  the  tree.  When  the  right  time  comes, 
they  all  return,  open  the  acorns  one  by  one,  devouring 
apparently  the  substance  of  the  nut,  and  probably  also  the 
grubs  of  beetles  which  have  developed  within.  When  the 
nuts  are  ripe,  again  they  return  to  the  same  tree  and  the 
same  process  is  repeated.  In  the  tree  figured  this  has  been 
noticed  each  year  since  1891. 

132.  Self-defense. — The  instinct  of  self-defense  is  even 
more  varied  in  its  manifestations.  It  may  show  itself 
either  in  the  impulse  to  make  war  on  an  intruder  or  in  the 
desire  to  flee  from  its  enemies.  Among  the  flesh-eating 
mammals  and  birds  fierceness  of  demeanor  serves  both  for 
the  securing  of  food  and  for  protection  against  enemies. 
The  stealthy  movements  of  the  lion,  the  skulking  habits  of 
the  wolf,  the  sly  selfishness  of  the  fox,  the  blundering  good- 
natured  power  of  the  bear,  the  greediness  of  the  hyena,  are 
all  proverbial,  and  similar  traits  in  the  eagle,  owl,  hawk, 
and  vulture  are  scarcely  less  matters  of  common  observa- 
tion. 

Herbivorous  animals,  as  a  rule,  make  little  direct  resist- 
ance to  their  enemies,  depending  rather  on  swiftness  of 
foot,  or  in  some  cases  on  simple  insignificance.  To  the  lat- 
ter cause  the  abundance  of  mice  and  mouse-like  rodents 
may  be  attributed,  for  all  are  the  prey  of  carnivorous  beasts 
and  birds,  and  even  snakes. 

Even  young  animals  of  any  species  show  great  fear  of 

their  hereditary  enemies.     The  nestlings  in  a  nest  of  the 

American  bittern  when  one  week  old  showed  no  fear  of 

man,  but  when  two  weeks  old  this  fear  was  very  manifest 

17 


246  ANIMAL  LIFE 

(Figs.  153  and  154).  Young  mocking-birds  will  go  into 
spasms  at  the  sight  of  an  owl  or  a  cat,  while  they  pay  little 
attention  to  a  dog  or  a  hen.  Monkeys  that  have  never 
seen  a  snake  show  almost  hysterical  fear  at  first  sight  of 
one,  and  the  same  kind  of  feeling  is  common  to  most 
men.  A  monkey  was  allowed  to  open  a  paper  bag  which 


1-  IG.  ioti.— Nestlings  of  the  American  bittern.  Two  of  a  brood  of  four  birds  one  week 
old,  at  which  age  they  showed  no  fear  of  man.  Photograph  by  E.  H.  TABOR, 
Meridian,  N.  Y.,  May  31,  1898.  (Permission  of  Macmillan  Company,  publishers  of 
Bird-Lore.) 

contained  a  live  snake.  He  was  staggered  by  the  sight, 
but  after  a  while  went  back  and  looked  in  again,  to  repeat 
the  experience.  Each  wild  animal  has  its  special  instinct 
of  resistance  or  method  of  keeping  off  its  enemies.  The 
stamping  of  a  sheep,  the  kicking  of  a  horse,  the  running 
in  a  circle  of  a  hare,  and  the  skulking  in  a  circle  of  some 
foxes,  are  examples  of  this  sort  of  instinct. 


INSTINCT  AND  REASON 


24T 


133.  Play. — The  play  instinct  is  developed  in  numerous 
animals.  To  this  class  belong  the  wrestlings  and  mimic 
fights  of  young  dogs,  bear  cubs,  seal  pups,  and  young 
beasts  generally.  Cats  and  kittens  play  with  mice.  Squir- 


FIG.  154.— Nestlings  of  the  American  bittern.  The  fonr  members  of  the  brood  of 
which  two  are  shown  in  Fig.  153.  two  weeks  old,  when  they  showed  marked  fear 
of  man.  Photograph  by  F.  M.  CHAPMAN,  Meridian,  N.  Y.,  June  8,  1898.  (Per- 
mission of  Macmillan  Company,  publishers  of  Bird-Lore.) 

rels  play  in  the  trees.  Perhaps  it  is  the  play  impulse  which 
leads  the  shrike  or  butcher-bird  to  impale  small  birds  and 
beetles  on  the  thorns  about  its  nest,  a  ghastly  kind  of  orna- 
ment that  seems  to  confer  satisfaction  on  the  bird  itself. 
The  talking  of  parrots  and  their  imitations  of  the  sounds 
they  hear  seem  to  be  of  the  nature  of  play.  The  greater 


248  ANIMAL  LIFE 

their  superfluous  energy  the  more  they  will  talk.  Much  of 
the  singing  of  birds,  and  the  crying,  calling,  and  howling  of 
other  animals,  are  mere  play,  although  singing  primarily  be- 
longs to  the  period  of  reproduction,  and  other  calls  and 
cries  result  from  social  instincts  or  from  the  instinct  to 
care  for  the  young. 

134.  Climate. — Climatic  instincts  are  those  which  arise 
from  the  change  of  seasons.     When  the  winter  comes  the 
fur-seal  takes  its  long  swim  to  the  southward;  the  wild 
geese  range  themselves  in  wedge-shaped  flocks  and  fly  high 
and  far,  calling  loudly  as  they  go ;  the  bobolinks  straggle 
away  one  at  a  time,  flying  mostly  in  the  night,  and  most  of 
the  smaller  birds  in  cold  countries  move  away  toward  the 
tropics.     All  these  movements  spring  from  the  migratory 
instinct.     Another  climatic  instinct  leads  the  bear  to  hide 
in  a  cave  or  hollow  tree,  where  he  sleeps  or  hibernates  till 
spring.     In  some  cases  the  climatic  instinct  merges  in  the 
homing  instinct  and  the  instinct  of  reproduction.     When 
the  birds  move  north  in  the  spring  they  sing,  mate,  and 
build  their  nests.     The  fur-seal  goes  home  to  rear  its  young. 
The  bear  exchanges  its  bed  for  its  lair,  and  its  first  business 
after  waking  is  to  make  ready  to  rear  its  young. 

135.  Environment. — Environmental    instincts     concern 
the  creature's  mode  of  life.    Such  are  the  burrowing  instincts 
of  certain  rodents,  the  woodchucks,  gophers,  and  the  like. 
To  enumerate  the  chief  phases  of  such  instincts  would  be 
difficult,  for  as  all  animals  are   related  to  their  environ- 
ment, this  relation  must  show  itself  in  characteristic  in- 
stincts. 

136.  Courtship.— The  instincts  of  courtship  relate  chiefly 
to  the  male,  the  female  being  more  or  less  passive.     Among 
many  fishes  the  male  struts  before  the  female,  spreading 
his  fins,  intensifying  his  pigmented  colors  through  muscu- 
lar tension,  and  in  such  fashion  as  he  can  makes  himself  the 
preferred  of  the  female.     In  the  little  brooks  in  spring 
male  minnows  can  be  found  with  warts  on  the  nose  or  head, 


INSTINCT  AND  REASON  249 

with  crimson  pigment  on  the  fins,  or  blue  pigment  on  the 
back,  or  jet-black  pigment  all  over  the  head,  or  with  varied 
combinations  of  all  these.  Their  instinct  is  to  display  all 
these  to  the  best  advantage,  even  though  the  conspicuous 
hues  lead  to  their  own  destruction.  Against  this  contin- 
gency Nature  provides  a  superfluity  of  males. 

Among  the  birds  the  male  in  spring  is  in  very  many 
species  provided  with  an  ornamental  plumage  which  he 
sheds  when  the  breeding  season  is  over.  The  scarlet,  crim- 
son, orange,  blue,  black,  and  lustrous  colors  of  birds  are 
commonly  seen  only  on  the  males  in  the  breeding  season, 
the  young  males  and  all  males  in  the  fall  having  the  plain 
brown  gray  or  streaky  colors  of  the  female.  Among  the 
singing  birds  it  is  chiefly  the  male  that  sings,  and  his  voice 
and  the  instinct  to  use  it  are  commonly  lost  when  the  young 
are  hatched  in  the  nest. 

Among  polygamous  mammals  the  male  is  usually  much 
larger  than  the  female,  and  his  courtship  is  often  a 
struggle  with  other  males  for  the  possession  of  the  female. 
Among  the  deer  the  male,  armed  with  great  horns,  fight 
to  the  death  for  the  possession  of  the  female  or  for  the 
mastery  of  the  herd.  The  fur-seal  has  on  an  average  a 
family  of  about  thirty-two  females  (Fig.  71),  and  for  the 
control  of  his  harem  others  are  ready  at  all  times  to  dispute 
the  possession.  But  with  monogamous  animals  like  the 
true  or  hair  seal  or  the  fox,  where  a  male  mates  with  a 
single  female,  there  is  no  such  discrepancy  in  size  and 
strength,  and  the  warlike  force  of  the  male  is  spent  on  out- 
side enemies,  not  on  his  own  species. 

137.  Reproduction. — The  movements  of  many  migra- 
tory animals  are  mainly  controlled  by  the  impulse  to  repro- 
duce. Some  pelagic  fishes,  especially  flying-fishes  and  fishes 
allied  to  the  mackerel,  swim  long  distances  to  a  region 
favorable  for  a  deposition  of  spawn.  Some  species  are 
known  only  in  the  waters  they  make  their  breeding  homes, 
the  individuals  being  scattered  through  the  wide  seas  at 


250  ANIMAL  LIFE 

other  times.  Many  fresh-water  fishes,  as  trout,  suckers, 
etc.,  forsake  the  large  streams  in  the  spring,  ascending  the 
small  brooks  where  they  can  rear  their  young  in  greater 
safety.  Still  others,  known  as  anadromous  fishes,  feed 
and  mature  in  the  sea,  but  ascend  the  rivers  as  the  im- 
pulse of  reproduction  grows  strong.  Among  such  species 
are  the  salmon,  shad,  alewife,  sturgeon,  and  striped  bass  in 
American  waters.  The  most  noteworthy  case  of  the  ana- 
dromous instinct  is  found  in  the  king  salmon  or  quinnat 
of  the  Pacific  coast.  This  great  fish  spawns  in  November. 
In  the  Columbia  River  it  begins  running  in  March  and 
April,  spending  the  whole  summer  in  the  ascent  of  the 
river  without  feeding.  By  autumn  the  individuals  are 
greatly  changed  in  appearance,  discolored,  worn,  and  distort- 
ed. On  reaching  the  spawning  beds,  some  of  them  a  thou- 
sand miles  from  the  sea,  the  female  deposits  her  eggs  in 
the  gravel  of  some  shallow  brook.  After  they  are  fertilized 
both  male  and  female  drift  tail  foremost  and  helpless  down 
the  stream,  none  of  them  ever  surviving  to  reach  the  sea. 
The  same  habits  are  found  in  other  species  of  salmon  of 
the  Pacific,  but  in  most  cases  the  individuals  of  other  spe- 
cies do  not  start  so  early  or  run  so  far.  A  few  species  of 
fishes,  as  the  eel,  reverse  this  order,  feeding  in  the  rivers 
and  brackish  creeks,  dropping  down  to  the  sea  to  spawn. 

The  migration  of  birds  has  relation  to  reproduction  as 
well  as  to  changes  of  weather.  As  soon  as  they  reach  their 
summer  homes,  courtship,  mating,  nest-building,  and  the 
care  of  the  young  occupy  the  attention  of  every  species. 

138.  Care  of  the  young.— In  the  animal  kingdom  one  of 
the  great  factors  in  development  has  been  the  care  of  the 
young.  This  feature  is  a  prominent  one  in  the  specializa- 
tion of  birds  and  mammals.  When  the  young  are  cared  for 
the  percentage  of  loss  in. the  struggle  for  life  is  greatly  re- 
duced, the  number  of  births  necessary  to  the  maintenance 
of  the  species  is  much  less,  and  the  opportunities  for  spe- 
cialization in  other  relations  of  life  are  much  greater. 


INSTINCT  AND  REASON  251 

In  these  regards,  the  nest-building  and  home-making 
animals  have  the  advantage  over  those  that  have  not  these 
instincts.  The  animals  that  mate  for  life  have  the  advan- 
tage over  polygamous  animals,  and  those  whose  social  or 
mating  habits  give  rise  to  a  division  of  labor  over  those 
with  instincts  less  highly  specialized. 

The  interesting  instincts  and  habits  connected  with  nest 
or  home  building  and  the  care  of  the  young  are  discussed 
in  the  next  chapter. 

139.  Variability  of  instincts. — When  we  study  instincts 
of  animals  with  care  and  in  detail,  we  find  that  their  regu- 
larity is  much  less  than  has  been  supposed.     There  is  as 
much  variation  in  regard  to  instinct  among  individuals  as 
there  is  with  regard  to  other  characters  of  the  species. 
Some  power  of  choice  is  found  in  almost  every  operation  of 
instinct.     Even  the  most  machine-like  instinct  shows  some 
degree  of  adaptability  to  new  conditions.     On  the  other 
hand,  in  no  animal  does  reason  show  entire  freedom  from 
automatism  or  reflex  action.     "  The  fundamental  identity 
of  instinct  with  intelligence,"  says  an  able  investigator,  "  is 
shown  in  their  dependence  upon  the  same  structural  mech- 
anism (the  brain  and  nerves)  and  in  their  responsive  adap- 
tability." 

140.  Reason. — Eeason  or  intellect,  as  distinguished  from 
instinct,  is  the  choice,  more  or  less  conscious,  among  re- 
sponses to  external  impressions.     Its  basis,  like  that  of  in- 
stinct, is  in  reflex  action.     Its  operations,  often  repeated, 
become  similarly  reflex  by  repetition,  and  are  known  as 
habit.     A  habit  is  a  voluntary  action  repeated  until  it  be- 
comes reflex.     It  is  essentially  like  instinct  in  all  its  mani- 
festations.    The  only  evident  difference  is  in  its  origin. 
Instinct  is  inherited.     Habit  is  the  reaction  produced  with- 
in the  individual  by  its  own  repeated    actions.      In  the 
varied  relations  of  life  the  pure  reflex  action  becomes  inade- 
quate.    The  sensorium  is  offered  a  choice  of  responses.    To 
choose  one  and  to  reject  the  others  is  the  function  of  intel- 


252  ANIMAL  LIFE 

lect  or  reason.  "While  its  excessive  development  in  man 
obscures  its  close  relation  to  instinct,  both  shade  off  by 
degrees  into  reflex  action.  Indeed,  no  sharp  line  can  be 
drawn  between  unconscious  and  subconscious  choice  of 
reaction  and  ordinary  intellectual  processes. 

Most  animals  have  little  self-consciousness,  and  their 
reasoning  powers  at  best  are  of  a  low  order ;  but  in  kind, 
at  least,  the  powers  are  not  different  from  reason  in  man. 
A  horse  reaches  over  the  fence  to  be  company  to  another. 
This  is  instinct.  When  it  lets  down  the  bars  with  its  teeth, 
that  is  reason.  When  a  dog  finds  its  way  home  at  night  by 
the  sense  of  smell,  this  may  be  instinct ;  when  he  drags  a 
stranger  to  his  wounded  master,  that  is  reason.  When  a 
jack-rabbit  leaps  over  the  brush  to  escape  a  dog,  or  runs  in 
a  circle  before  a  coyote,  or  when  it  lies  flat  in  the  grass  as  a 
round  ball  of  gray  indistinguishable  from  grass,  this  is  in- 
stinct. But  the  same  animal  is  capable  of  reason — that  is, 
of  a  distinct  choice  among  lines  of  action.  Not  long  ago  a 
rabbit  came  bounding  across  the  university  campus  at  Palo 
Alto.  As  it  passed  a  corner  it  suddenly  faced  two  hunting 
dogs  running  side  by  side  toward  it.  It  had  the  choice  of 
turning  back,  its  first  instinct,  but  a  dangerous  one ;  of 
leaping  over  the  dogs,  or  of  lying  flat  on  the  ground.  It 
chose  none  of  these,  and  its  choice  was  instantaneous.  It 
ceased  leaping,  ran  low,  and  went  between  the  dogs  just  as 
they  were  in  the  act  of  seizing  it,  and  the  surprise  of  the 
dogs,  as  they  stopped  and  tried  to  hurry  around,  was  the 
same  feeling  that  a  man  would  have  in  like  circumstances. 

On  the  open  plains  of  Merced  County,  California,  the 
jack-rabbit  is  the  prey  of  the  bald  eagle.  Not  long  since  a 
rabbit  pursued  by  an  eagle  was  seen  to  run  among  the 
cattle.  Leaping  from  cow  to  cow,  he  used  these  animals 
as  a  shelter  from  the  savage  bird.  When  the  pursuit  was 
closer,  the  rabbit  broke  cover  for  a  barbed  wire  fence. 
When  the  eagle  swooped  down  on  it,  the  rabbit  moved  a 
few  inches  to  the  right,  and  the  eagle  could  not  reach  him 


INSTINCT  AND  REASON  253 

through  the  fence.  When  the  eagle  came  down  on  the 
other  side,  he  moved  across  to  the  first.  And  this  was  con- 
tinued until  the  eagle  gave  up  the  chase.  It  is  instinct 
that  leads  the  eagle  to  swoop  on  the  rabbit.  It  is  instinct 
again  for  the  rabbit  to  run  away.  But  to  run  along  the  line 
of  a  barbed  wire  fence  demands  some  degree  of  reason.  If 
the  need  to  repeat  it  arose  often  in  the  lifetime  of  a  single 
rabbit  it  would  become  a  habit. 

The  difference  between  intellect  and  instinct  in  lower 
animals  may  be  illustrated  by  the  conduct  of  certain  mon- 
keys brought  into  relation  with  new  experiences.  At  one 
time  we  had  two  adult  monkeys,  "  Bob  "  and  "  Jocko,"  be- 
longing to  the  genus  Macacus.  Neither  of  these  possessed 
the  egg-eating  instinct.  At  the  same  time  we  had  a  baby 
monkey,  "  Mono,"  of  the  genus  Cercopithecus.  Mono  had 
never  seen  an  egg,  but  his  inherited  impulses  bore  a  direct 
relation  to  feeding  on  eggs,  just  as  the  heredity  of  Macacus 
taught  the  others  how  to  crack  nuts  or  to  peel  fruit. 

To  each  of  these  monkeys  we  gave  an  egg,  the  first  that 
any  of  them  had  ever  .seen.  The  baby  monkey,  Mono, 
being  of  an  egg-eating  race,  devoured  his  egg  by  the  opera- 
tion of  instinct  or  inherited  habit.  On  being  given  the 
egg  for  the  first  time,  he  cracked  it  against  his  upper  teeth, 
making  a  hole  in  it,  and  sucked  out  all  the  substance. 
Then  holding  the  egg-shell  up  to  the  light  and  seeing  that 
there  was  no  longer  anything  in  it,  he  threw  it  away.  All 
this  he  did  mechanically,  automatically,  and  it  was  just  as 
well  done  with  the  first  egg  he  ever  saw  as  with  any  other 
he  ate.  All  eggs  since  offered  him  he  has  treated  in  the 
same  way. 

The  monkey  Bob  took  the  egg  for  some  kind  of  nut. 
He  broke  it  against  his  upper 'teeth  and  tried  to  pull  off 
the  shell,  when  the  inside  ran  out  and  fell  on  the  ground. 
He  looked  at  it  for  a  moment  in  bewilderment,  took  both 
hands  and  scooped  up  the  yolk  and  the  sand  with  which  it 
was  mixed  and  swallowed  the  whole.  Then  he  stuffed  the 


254  ANIMAL   LIFE 

shell  itself  into  his  mouth.  This  act  was  not  instinctive. 
It  was  the  work  of  pure  reason.  Evidently  his  race  was 
not  familiar  with  the  use  of  eggs  and  had  acquired  no  in- 
stincts regarding  them.  He  would  do  it  better  next  time. 
Eeason  is  an  inefficient  agent  at  first,  a  weak  tool;  but 
when  it  is  trained  it  becomes  an  agent  more  valuable  and 
more  powerful  than  any  instinct. 

The  monkey  Jocko  tried  to  eat  the  egg  offered  him  in 
much  the  same  way  that  Bob  did,  but,  not  liking  the  taste, 
he  threw  it  away. 

The  confusion  of  highly  perfected  instinct  with  intellect 
is  very  common  in  popular  discussions.  Instinct  grows 
weak  and  less  accurate  in  its  automatic  obedience  as  the 
intellect  becomes  available  in  its  place.  Both  intellect  and 
instinct  are  outgrowths  from  the  simple  reflex  response  to 
external  conditions.  But  instinct  insures  a  single  definite 
response  to  the  corresponding  stimulus.  The  intellect  has 
a  choice  of  responses.  In  its  lower  stages  it  is  vacillating 
and  ineffective ;  but  as  its  development  goes  on  it  becomes 
alert  and  adequate  to  the  varied  conditions  of  life.  It 
grows  with  the  need  for  improvement.  It  will  therefore 
become  impossible  for  the  complexity  of  life  to  outgrow 
the  adequacy  of  man  to  adapt  himself  to  its  conditions. 

Many  animals  currently  believed  to  be  of  high  intelli- 
gence are  not  so.  The  fur-seal,  for  example,  finds  it  way 
back  from  the  long  swim  of  two  or  three  thousand  miles 
through  a  foggy  and  stormy  sea,  and  is  never  too  late  or  too 
early  in  arrival.  The  female  fur-seal  goes  two  hundred 
miles  to  her  feeding  grounds  in  summer,  leaving  the  pup 
on  the  shore.  After  a  week  or  two  she  returns  to  find  him 
within  a  few  rods  of  the  rocks  where  she  had  left  him. 
Both  mother  and  young  know  each  other  by  call  and  by 
odor,  and  neither  is  ever  mistaken,  though  ten  thousand 
other  pups  and  other  mothers  occupy  the  same  rookery. 
But  this  is  not  intelligence.  It  is  simply  instinct,  because 
it  has  no  element  of  choice  in  it.  Whatever  its  ancestors 


INSTINCT  AND  REASON  255 

were  forced  to  do  the  fur-seal  does  to  perfection.  Its  in- 
stincts are  perfect  as  clockwork,  and  the  necessities  of 
migration  must  keep  them  so.  But  if  brought  into  new 
conditions  it  is  dazed  and  stupid.  It  can  not  choose  when 
different  lines  of  action  are  presented. 

The  Berirg  Sea  Commission  once  made  an  experiment 
on  the  possibility  of  separating  the  young  male  fur-seals, 
or  "  killables,"  from  the  old  ones  in  the  same  band.  The 
method  was  to  drive  them  through  a  wooden  chute  or  run- 
way with  two  valve-like  doors  at  the  end.  These  animals  can 
be  driven  like  sheep,  but  to  sort  them  in  the  way  proposed 
proved  impossible.  The  most  experienced  males  would 
beat  their  noses  against  a  closed  door,  if  they  had  seen  a 
seal  before  them  pass  through  it.  That  this  door  had  been 
shut  and  another  opened  beside  it  passed  their  comprehen- 
sion. They  could  not  choose  the  new  direction.  In  like 
manner  a  male  fur-seal  will  watch  the  killing  and  skinning 
of  his  mates  with  perfect  composure.  He  will  sniff  at  their 
blood  with  languid  curiosity  ;  so  long  as  it  is  not  his  own 
it  does  not  matter.  That  his  own  blood  may  flow  out  on 
the  ground  in  a  minute  or  two  he  can  not  foresee. 

Eeason  arises  from  the  necessity  for  a  choice  among  ac- 
tions. It  may  arise  as  a  clash  among  instincts  which  forces 
on  the  animal  the  necessity  of  choosing.  A  doe,  for  ex- 
ample, in  a  rich  pasture  has  the  instinct  to  feed.  It  hears 
the  hounds  and  has  the  instinct  to  flee.  Its  fawn  may  be 
with  her  and  it  is  her  instinct  to  remain  and  protect  it. 
This  may  be  done  in  one  of  several  ways.  In  proportion  as 
the  mother  chooses  wisely  will  be  the  fawn's  chance  of  sur- 
vival. Thus  under  difficult  conditions,  reason  or  choice 
among  actions  rises  to  the  aid  of  the  lower  animals  as  well 
as  man. 

141.  Mind. — The  word  mind  is  popularly  used  in  two 
different  senses.  In  the  biological  sense  the  mind  is  the 
collective  name  for  the  functions  of  the  sensorium  in  men 
and  animals.  It  is  the  sum  total  of  all  psychic  changes, 


256  ANIMAL  LIFE 

actions  and  reactions.  Under  the  head  of  psychic  functions 
are  included  all  operations  of  the  nervous  system  as  well  as 
all  functions  of  like  nature  which  may  exist  in  organisms 
without  specialized  nerve  fibers  or  nerve  cells.  As  thus  de- 
fined mind  would  include  all  phenomena  of  irritability,  and 
even  plants  have  the  rudiments  of  it.  The  operations  of 
the  mind  in  this  sense  need  not  be  conscious.  With  the 
lower  animals  almost  all  of  them  are  automatic  and  uncon- 
scious. With  man  most  of  them  must  be  so.  All  func- 
tions of  the  sensorium,  irritability  ^reflex  action,  instinct, 
reason,  volition,  are  alike  in  essential  nature  though  differ- 
ing greatly  in  their  degree  of  specialization. 

In  another  sense  the  term  mind  is  applied  only  to  con- 
scious reasoning  or  conscious  volition.  In  this  sense  it  is 
mainly  an  attribute  of  man,  the  lower  animals  showing  it 
in  but  slight  degree.  The  discussion  as  to  whether  lower 
animals  have  minds  turns  on  the  definition  of  mind,  and 
our  answer  to  it  depends  on  the  definition  we  adopt. 


FIG.  155.— A  "pointer"  dog  in  the  act  of  "pointing,"  a  specialized  instinct. 
(Permission  of  G.  O.  Shields,  publisher  of  Recreation.) 


CHAPTER  XV 

HOMES    AND    DOMESTIC    HABITS 

142.  Importance  of  care  of  the  young. — The  nest-building 
and  domestic  habits  of^nimals  are  adaptations,  but  adapta- 
tions of  behavior  or  haoit  rather  than  of  structure,  and  are 
based  on  instinct,  intelligence,  and  reason.    These  instincts 
and  habits  are  among  the  most  important  shown  by  animals, 
because  on  them  depends  largely  the  continuance  of  the 
species.     Of  primary  importance  in  the  perpetuation  of  the 
species  is  the  possession  by  animals  of  adaptations  of  struc- 
ture and  behavior,  which  help  the  individual  live  long  enough 
to  attain  full  development  and  to  leave  offspring.     But  in 
the  case  of  many  animals  a  successful  start  in  life  on  the 
part  of  the  offspring  'depends  on  the  existence  in  the  par- 
ents of  certain  domestic  habits  concerned  with  the  care  and 
protection  of  their  "young.    The  young  of  many  animals  de- 
pend absolutely,  for  a  part  of  their  lifetime,  on  this  parental 
care.    In  these  cases'it  is  quite  as  necessary  for  the  continued 
existence  of  the  species  that  the  habits  that  afford  this  care 
be  successful  as  that  the  parent  should  come  successfully  to 
mature  development  and  to  the  production  of  offspring. 

143.  Care  of  the  young,  and  communal  life. — The  nest- 
building  or  home-making  habits  and  the  continued  per- 
sonal care  of  the  young  for  a  part  of  their  lifetime  are  most 
highly  developed  and  widespread  among  the  birds,  mam- 
mals, and  insects ;  and  it  is  both  among  the  insects  and 
the  higher  vertebrates  that  we  find  most  developed  those 
social  and  communal  habits  discussed  in  Chapter  IX.     The 
principal  activities  of  an  animal  community  have  to  do 

18  257 


258  ANIMAL  LIFE 

with  the  protection  and  sustenance  of  the  young,  and  the 
care  of  the  young  is  undoubtedly  a  chief  factor  in  the  de- 
velopment of  communal  life. 

144.  The  invertebrates  (except  spiders  and  insects). — 
Among  the  lower  invertebrates  parental  aid  to  the  young  is 
confined  almost  exclusively  to  exhibitions  of  pure  instinct. 
And  this  is  true  of  many  of  the  higher  animals  also.  Eggs 
are  deposited  in  sheltered  places,  and  in  such  places  and 
under  such  circumstances  that  the  young  on  hatching  will 
find  themselves  close  to  a  supply  of  their  natural  food.  The 
depositing  of  eggs  in  water  by  par^ts  with  terrestrial  hab- 
its whose  young  are  aquatic,  is  an  example.  The  toad, 
which  lives  on  land,  feeding  on  insects,  has  young  which 
live  in  water  and  feed  on  minute  aquatic  plants  (algae). 
The  dragon  fly,  that  hawks  over  the  pond  or  brook  with 
glistening  wings,  has  young  that  crawl  in  the  slime  and 
mud  at  the  bottom  of  the  pool.  With  most  animals,  after 
laying  eggs,  the  parents  show  no  further  solicitude  toward 
their  offspring.  The  eggs  are  left  to  the  vicissitudes  of 
fortune,  and  the  parents  know  nothing  of  their  fate.  In 
many  cases  the  parent  dies  before  the  young  are  hatched. 
The  mammals  and  birds  are  the  only  two  great  groups  ex- 
cepted,  although  there  are  numerous  specific  exceptions 
scattered  among  the  lower  invertebrates,  fishes,  batrachians, 
and  higher  invertebrates,  notably  the  insects. 

There  are  no  instances  of  care  of  the  young  after  hatch- 
ing among  the  sponges,  polyps,  worms,  or  star-fishes  and 
sea-urchins,  and  but  few  among  the  crustaceans  and  mol- 
lusks.  But  there  are  in  some  of  these  groups  a  few  cases 
of  nest  or  home  building  in  a  crude  and  simple  way.  Cer- 
tain sea-urchins  (Fig.  156)  and  worms  and  mollusks  bore 
into  stones,  and  remain  in  the  shelter  afforded  by  the  cav- 
ity. A  shell-fish  (Lima  Mams)  cements  together  bits  of 
coralline,  shells,  and  sand  to  form,  a  crude  nest  or  hiding- 
place.  The  cray-fish  digs  a  cylindrical  burrow  in  the  ground 
in  which  it  can  hide. 


HOMES  AND  DOMESTIC  HABITS  259 

145.  Spiders. — Most  spiders  spin  silken  cocoons  or  sacs 
in  which  to  deposit  their  eggs.  Some  spiders  carry  this 
egg-filled  cocoon  about  with  them  for  the  sake  of  protect- 
ing the  eggs.  After  hatching,  the  spiderlings  remain  in  the 
cocoon  a  short  time,  feeding  on  each  other !  Thus  only  the 


FIG.  15G. — Sea-urchins  living  in  holes  bored  into  rocks  of  the  seashore  below  high- 
tide  line. 

strongest  survive  and  issue  from  the  cocoon  to  earn  their 
living  in  the  outer  world.  With  certain  species  of  spiders 
the  young  after  hatching  leave  the  cocoon  and  gather  on 
the  back  of  the  mother  and  are  carried  about  by  her  for 
some  time.  In  connection  with  their  webs  or  snares  many 
spiders  have  silken  tunnels  or  tubes  in  which  to  lie  hidden, 
a  sort  of  sheltering  nest.  Those  spiders  that  live  on  the 
ground  make  for  themselves  cylindrical  burrows  or  holes 
in  the  ground,  usually  lined  with  silk,  in  which  they  hide 
when  not  hunting  for  food.  Especially  interesting  among 
the  many  kinds  of  these  spider  nests  are  the  burrows  of 
the  various  trap-door  spiders.  These  spiders  are  common 
in  California  and  some  other  far  Western  States.  The  bur- 


2CO  ANIMAL  LIFE 

row  (Fig.  157)  or  cylindrical  hole  is  closed  above  by  a  silken, 
thick,  hinged  lid  or  door,  a  little  larger  in  diameter  than 
the  hole  and  neatly  beveled  on  the  edge,  so  as  to  fit  tightly 
into  and  perfectly  cover  the  hole  when  closed.  The  upper 
surface  of  the  door  is  covered  with  soil,  bits  of  leaves,  and 
wood,  so  as  to  resemble  very  exactly  the  ground  surface 
about  it.  We  have  found  these  trap-door  nests  in  Cali- 
fornia in  moss-covered  ground,  and  here  the  lids  of  the  nests 
were  always  covered  with  green,  growing  moss. 

An  English  naturalist  who  studied  the  habits  of  these 
trap-door  spiders  found  that  if  he  removed  the  soil  and  bits 
of  bark  and  twigs,  or  the  inoss,  from  the  upper  surface  of 
the  lid  the  spider  always  re-covered  it.  It  is,  of  course, 
plain  that  by  means  of  this  covering  the  nest  is  perfectly 
concealed,  the  surface  of  the  closed  door  not  being  dif- 
ferent from  the  surrounding  ground  surface.  This  natu- 
ralist finally  removed  the  moss  not  only  from  the  surface 
of  a  trap-door,  but  also  from  all  the  ground  in  a  circle  of  a 
few  feet  about  the  nest.  The  next  day  he  found  that  the 
spider  had  brought  moss  from  outside  the  cleared  space 
and  covered  the  trap-door  with  it,  thus  making  it  very  con- 
spicuous in  the  cleared  ground  space.  The  spider's  instinct 
was  not  capable  of  that  quick  modification  to  allow  it  to  do 
what  a  reasoning  animal  would  have  done — namely,  cov- 
ered the  trap-door  only  with  soil  to  make  it  resemble  the 
cleared  ground  about  it. 

Another  interesting  nest-making  spider  is  the  turret- 
spider,  that  builds  up  a  little  tower  (Fig.  158)  of  sticks  and 
soil  and  moss  above  its  burrow.  The  sticks  of  which  this 
burrow  are  built  are  an  inch  or  two  in  length,  and  are 
arranged  in  such  manner  as  make  the  turret  five-sided. 
The  sticks  are  fastened  together  with  silk,  and  the  turret 
is  made  two  or  three  inches  high.  This  turret-building 
spider  is  one  of  those  that  carry  about  their  egg-cocoon 
with  them.  A  female  of  this  spider  in  captivity  was  ob- 
served to  pay  much  attention  to  caring  for  this  cocoon. 


262 


ANIMAL  LIFE 


"  If  the  weather  was  cold  or  damp,  she  retired  to  her  tunnel ; 
but  if  the  jar  in  which  she  lived  was  set  where  the  sun 

could  shine  upon  it,  she  soon  re- 
appeared and  allowed  the  cocoon 
to  bask  in  the  sunlight.     If  the 
jar  was  placed  near  a  stove  that 
had  a  fire  in  it,  the  cocoon  was 
put  on  the  side  next  the  source 
of   warmth ;   if   the    jar 
was  turned  around,  she 
lost  no  time   in  moving 
the  cocoon  to  the  warmer 
side.     Two  months  after 
the   eggs   were  laid  the 
young  spiders  made  their 
appearance  and  immediately 
perched  upon  their  mother,  many 
on  her  back,  some  on  her  head, 
and  even  on  her  legs.     She  car- 
ried them  about  with  her  and  fed 
them,  and  until  they  were  older 
they  never  left  their  mother  for 
a  moment." 

146.  Insects. — So  much  space 
has  already  been  devoted  to  an 
account  of  the  elaborate  nest-making  and  domestic  habits 
of  the  bees,  ants,  and  termites  (see  Chapter  IX),  that  we 
need  in  this  place  merely  refer  to  that  account.  It  is 
among  these  social  insects  that  the  most  interesting  and 
highly  specialized  habits  connected  with  the  care  of  the 
young  and  the  building  of  homes  are  found. 

Many  insects  make  for  themselves  simple  burrows  or 
nests  in  the  ground  or  in  wood.  The  young  or  larvas  of 
certain  moths  burrow  about  in  the  soft  inside  tissue  of 
leaves,  and  the  whole  life  of  the  moth  except  its  short  adult 
stage  is  passed  inside  the  leaf.  These  larvae  are  called  leaf- 


FIQ.  158.— Nest  of  the  turret- 
spider. 


HOMES  AND  DOMESTIC  HABITS  263 

miners.  The  larvae  of  some  moths  and  of  many  hymenop- 
terous  insects  live  in  galls  on  live  plants.  These  galls  are 
simply  abnormal  growths  of  plant  tissue,  and  are  caused  by 
the  irritating  effect  on  the  tissue  of  the  larvae  which  hatch 
from  eggs  that  have  been  thrust  into  the  soft  plant  sub- 
stance by  the  female  insects.  In  the  familiar  galls  on  the 
golden-rod  live  the  larvae  of  a  small  moth,  and  in  the  vari- 
ous kinds  of  oak  galls  live  the  young  of  the  numerous  spe- 
cies of  Cynipidce,  the  hymenopterous  gall  insects.  The  tiny 
larvae  of  some  of  the  midges  live  in  small  galls  on  various 
plants.  To  this  last  group  of  gall-making  insects  belongs 
the  dreaded  Hessian  fly,  the  most  destructive  insect  pest  of 
wheat. 

Among  the  bees  and  wasps  only  a  few  species,  compara- 
tively, are  communal  or  live  in  communities.  But  nearly 
all  the  wasps  and  bees,  whether  social  or  solitary  in  habit, 
build  nests  for  their  young  and  provide  the  young  with 
food,  either  by  storing  it  in  the  nest  or  by  hunting  for  it 
and  bringing  it  to  the  nest  as  long  as  the  young  are  in  the 
larval  condition.  The  "mud-daubers"  or  thread-waisted 
wasps  make  nests  of  mud  attached  to  the  lower  surface  of 
flat  stones,  to  the  ceiling  of  buildings,  or  in  other  out-of- 
the-way  and  safe  places.  These  nests  usually  have  the  form 
of  several  tubes  an  inch  or  so  long  placed  side  by  side.  In 
each  of  the  tubes  or  cells  an  egg  is  laid,  and  with  it  a 
spider  which  has  been  stung  so  as  to  be  paralyzed  but 
not  killed.  When  the  young  wasp  hatches  from  the  egg 
as  a  grub  or  larva,  it  feeds  on  the  unfortunate  spider. 
Others  of  the  solitary  wasps  make  nests  in  the  ground 
or  in  wood,  and  all  of  them  provision  their  nests  with 
some  particular  kind  of  insect  or  spider.  Some  use  only 
caterpillars,  some  plant-lice,  and  some  grasshoppers.  Simi- 
larly the  solitary  bees  make  nests  in  the  ground  as  do  the 
mining-bees,  or  in  wood  as  do  the  carpenter-bees,  or  by 
cutting  and  fastening  together  leaves,  as  do  the  leaf -cutting 
bees.  The  bees  provision  their  nests,  not  with  paralyzed 


264  ANIMAL  LIFE 

insects,  but  with  masses  of  pollen  or  pollen  mixed  with 
nectar. 

147.  The  vertebrates. — It  is  among  the  vertebrates,  espe- 
cially in  the  higher  groups,  the  birds  and  mammals,  that 
we  find  the  care  of  the  young  most  perfectly  undertaken 
and  most  widespread. 

Among  the  fishes,  the  lowest  of  the  vertebrates,  most 
species  content  themselves  with  the  laying  of  many  eggs  in 
a  situation  best  suited  for  their  safe  hatching.  But  some 
species  show  interesting  domestic  habits.  The  female  cat- 
fish swims  about  with  her  brood,  much  as  a  hen  moves 
about  with  her  chickens.  Some  of  the  larger  ocean  cat- 
fish of  the  tropics  receive  the  eggs  or  the  young  within  the 
mouth  for  safety  in  time  of  danger.  Certain  sunfishes  care 
for  their  young,  keeping  them  together  in  still  places  in  the 
brook.  They  also  make  some  traces  of  a  nest,  which  the 
male  defends.  The  male  salmon  scoops  out  gravel  to  make 
a  shallow  nest,  in  which  the  female  deposits  her  eggs.  The 
male  then  covers  the  eggs.  The  males  of  the  species  of 
pipe-fish  and  sea-horses  receive  the  eggs  of  the  female  into 
a  groove  or  sac  between  the  folds  of  skin  on  the  lower  part 
of  the  tail.  Here  they  are  kept  until  the  little  fishes  are 
large  enough  to  swim  about  for  themselves.  The  brave 
little  sticklebacks  build  tiny  nests  about  an  inch  and  a  half 
or  two  inches  in  diameter,  with  a  small  opening  at  the  top. 
The  eggs  are  laid  in  this  nest,  and  the  young  fish  remain  in 
it  some  time  after  hatching.  The  male  parent  jealously 
guards  the  nest,  and  fights  bravely  with  would-be  intruders. 

The  batrachians  and  reptiles  rarely  show  any  care  for 
their  young.  The  eggs  of  most  batrachians  are  laid  in  the 
water  and  left  by  the  female.  The  males  of  the  Surinam 
toad  receive  the  eggs  in  pits  of  the  spongy  skin  of  the  back, 
where  they  remain  until  the  young  hatch.  The  eggs  of 
snakes  are  laid  under  logs  or  buried  in  the  sand,  and  no 
further  attention  is  given  them  by  the  parents. 

Among  the  birds,  on  the  other  hand,  nest-building  and 


HOMES  AND  DOMESTIC  HABITS 


265 


care  of  the  young  are  the  rule,  and  a  high  degree  of  devel- 
opment in  these  habits  is  reached.  All  of  us  are  familiar 
with  many  different  kinds  of  nests,  from  the  few  twigs 
loosely  put  together  by  the  mourning-dove  to  the  firm, 
closely  knit,  wool  or  feather  lined  nest  of  the  humming- 
bird (Fig.  159),  and  the  basket-like  hanging  nest  of  the 


FIG.  159.— Nest  and  eggs  of  the  Rufui*  humming-bird  (Trochilus  rufuf).    Photograph 
by  J.  O.  SNYDER,  Stanford  University,  California. 


2CG 


ANIMAL  LIFE 


oriole  (Fig.  161).  Not  all  birds  make  nests.  On  the  rocky 
islets  of  the  northern  oceans,  where  thousands  of  puffins 
and  auks  and  other  maritime  birds  gather  to  breed,  the 
eggs  are  laid  on  the  bare  rock.  At  the  other  extreme  is 
the  tailor  bird  of  India,  which  sews  together  leaves  by 
means  of  fibrous  strips  plucked  from  a  growing  plant  to 


FIG.  160.— Nest  and  young  of  the  Rufus  humming-bird  (Trocfiilus  rufus). 
by  J.  O.  SNYDEB,  Stanford  University,  California.' 


HOMES  AND   DOMESTIC  HABITS 


267 


FIG.  161.— Baltimore  orioles  and  nest ;  the  male  in  upper  left-hand  corner  of  figure. 

form  a  long,  bag-like  nest  (Fig.  162).  In  the  degree  of 
care  given  the  nestlings  there  is  also  much  difference.  The 
robin  brings  food  to  the  helpless  young  for  many  days,  and 


268 


ANIMAL 


finally  teaches  it  to  fly  and  to  hunt  for  food  for  itself. 
Young  chickens  are  not  so  helpless  as  the  nestling  robins, 


but  are  able  to  run  about,  and 
care  of    the   hen  mother  to 


under  the  guiding 
pick  up  food  for 
themselves. 

Among  the  mam- 
mals the  young  are 
always  given  some 
degree  of  care.  Ex- 
cepting in  the  case 
of  the  duck-bills,  the 
lowest  of  the  mam- 
mals, the  young  are 

»\       %£3  K^V          k°rn   alive — that  is, 

A  ~  B5ff    ,        are  not  hatched  from 

eggs  laid  outside  the 
body— and  are  nour- 
ished after  birth  for 
a  shorter  or  longer 
time  with  milk 
drawn  from  the 
body  of  the  mother. 
Before  birth  the 
young  undergoes  a 
longer  or  shorter 
period  of  development  and  growth  in  the  body  of  the 
mother,  being  nourished  by  the  blood  of  the  mother.  The 
nests  or  homes  of  mammals  present  varying  degrees  of 
elaborateness,  from  a  simple  cave-like  hole  in  the  rocks 
or  ground  to  the  elaborately  constructed  villages  of  the 
beavers  with  their  dams  and  conical  several-storied  houses 
(Fig.  163).  The  wood-rat  piles  together  sticks  and  twigs 
in  what  seems,  from  the  outside,  a  most  haphazard  fashion, 
but  which  results  in  the  construction  of  a  convenient  and 
ingenious  nest.  The  moles  and  pocket-gophers  (Fig.  165) 
build  underground  nests  composed  of  chambers  and  gal- 


FlG. 


2.— Tailor-bird  (Ornithotomus  sutorius) 
and  nest. 


270 


ANIMAL  LIFE 


FIG.  164.— Nest  of  the  California!!  bnph-tit  (Psaltriparus  minimus).    Photograph  by 
G.  O.  SNYDER,  Stanford  University,  California. 

leries.      The  prairie-dogs  make  burrows  in  groups,  forming 
large  villages. 

The  devotion  to  their  young  displayed  by  birds   and 
mammals  is  familiar  to  us.     The  parents  will  often  risk  or 


HOMES  AND  DOMESTIC  HABITS 


271 


suffer  the  loss  of  their  own  lives  in  protecting  their  off 
spring  from  enemies.  Many  mother  birds  have  the  instinct 
to  nutter  about  a  discovered  nest  crying  and  apparently 
broken-winged,  thus  leading  the  predatory  fox  or  weasel  to 


FIG.  165.— Nest  and  run-way  of  the  p6cket-gopher. 

fix  his  attention  on  the  mother  and  to  leave  the  nest  un- 
harmed. This  development  of  parental  care  and  protec- 
tion of  the  young  reaches  its  highest  degree  in  the  case  of 
the  human  species.  The  existence  of  the  family,  which  is 
the  unit  of  human  society,  rests  on  this  high  development 
of  care  for  the  young. 


CHAPTER  XVI 

GEOGRAPHICAL   DISTRIBUTION   OF   ANIMALS 

148.  Geographical  distribution. — Under  the  head  of  dis- 
tribution we  consider  the  facts  of  the  diffusion  of  organ- 
isms over  the  surface  of  the  earth,  and  the  laws  by  which 
this  diffusion  is  governed. 

The  geographical  distribution  of  animals  is  often  known 
as  zoogeography.  In  physical  geography  we  may  prepare 
maps  of  the  earth  which  shall  bring  into  prominence  the 
physical  features  of  its  surface.  Such  maps  would  show 
here  a  sea,  here  a  plateau,  here  a  range  of  mountains, 
there  a  desert,  a  prairie,  a  peninsula,  or  an  island.  In  po- 
litical geography  the  maps  show  the  physical  features  of 
the  earth,  as  related  to  the  states  or  powers  which  claim 
the  allegiance  of  the  people.  In  zoogeography  the  realms 
of  the  earth  are  considered  in  relation  to  the  types  or 
species  of  animals  which  inhabit  them.  Thus  a  series  of 
maps  of  the  United  States  could  be  drawn  which  would 
show  the  gradual  disappearance  of  the  buffalo  before  the 
attacks  of  man.  Another  might  be  drawn  which  would 
show  the  present  or  past  distribution  of  the  polar  bear, 
black  bear,  and  grizzly.  Still  another  might  show  the 
original  range  of  the  wild  hares  or  rabbits  of  the  United 
States,  the  white  rabbit  of  the  Northeast,  the  cotton-tail  of 
the  East  and  South,  the  jack-rabbit  of  the  plains,  the  snow- 
shoe  rabbit  of  the  Columbia  River,  the  tall  jack-rabbit  of 
California,  the  black  rabbits  of  the  islands  of  Lower  Cali- 
fonia,  and  the  marsh-hare  of  the  South  and  the  water-hare 
of  the  canebrakes,  and  that  of  all  their  relatives.  Such  a 
272 


FIG.  166.— Map  showing  the  distribution  of  the  clouded  Skipper  butterfly  (Lerema 
accius)  In  the  United  States.  The  butterfly  is  found  in  that  part  of  the  country 
shaded  in  the  map,  a  warm  and  moist  region.— After  SCUDDEK. 


Fio.  167.— Map  showing  the  distribution  of  the  Canadian  Skipper  butterfly  (Erynnis 
manitoba)  in  the  United  States.  The  butterfly  is  found  in  that  part  of  the 
country  shaded  in  the  map.  This  butterfly  is  subarctic  and  subalpine  in  dis- 
tribution, being  found  only  far  north  or  on  hicrh  mountains,  the  two  southern 
projecting  parts  of  its  range  being  in  the  Rocky  Mountains  and  in  the  Sierra 
Nevada  Mountains.— After  SCUDDKB. 
19 


274  ANIMAL  LIFE 

map  is  very  instructive,  and  it  at  once  raises  a  series  of 
questions  as  to  the  reasons  for  each  of  the  facts  in  geo- 
graphical distribution,  for  it  is  the  duty  of  science  to  sup- 
pose that  none  of  these  facts  is  arbitrary  or  meaningless. 
Each  fact  has  some  good  cause  behind  it. 

149.  Laws  of  distribution. — The  laws  governing  the  dis- 
tribution of  animals   are  reducible  to  three  very  simple 
propositions.     Every  species  of  animal  is  found  in  every  part 
of  the  earth  having  conditions  suitable  for  its  maintenance, 
unless — 

(a)  Its  individuals  have  been  unable  to  reach  this  re- 
gion, through  barriers  of  some  sort ;  or — 

(b)  Having  reached  it,  the  species  is  unable  to  maintain 
itself,  through   lack  of  capacity  for  adaptation,  through 
severity  of  competition  with  other  forms,  or  through  de- 
structive conditions  of  environment ;  or — 

(c)  Having  entered  and  maintained  itself,  it  has  become 
so  altered  in  the  process  of  adaptation  as  to  become  a  spe- 
cies distinct  from  the  original  type. 

150.  Species  debarred  by  barriers. — As  examples  of  the 
first  class  we  may  take  the  absence  of  kingbirds  or  meadow- 
larks  or  coyotes  in  Europe,  the  absence  of  the  lion  and 
tiger  in  South  America,  the  absence  of  the  civet-cat  in  New 
York,  and  that  of  the  bobolink  or  the  Chinese  flying-fox  in 
California.     In  each  of  these  cases  there  is  no  evident  rea- 
son why  the  species  in  question  should  not  maintain  itself 
if  once  introduced.     The  fact  that  it  does  not  exist  is,  in 
general,  an  evidence  that  it  has  never  passed  the  barriers 
which  separate  the  region   in   question   from  its  original 
home. 

Local  illustrations  of  the  same  kind  may  be  found  in 
most  mountainous  regions.  In  the  Yosemite  Valley  in 
California,  for  example,  the  trout  ascend  the  Merced  Eiver 
to  the  base  of  a  vertical  fall.  They  can  not  rise  above  this, 
and  so  the  streams  and  lakes  above  this  fall  are  destitute 
of  fish. 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS       275 

151.  Species  debarred  by  inability  to  maintain  their  ground. 

— Examples  of  the  second  class  are  seen  in  animals  which 
man  has  introduced  from  one  country  to  another.  The 
nightingale,  the  starling,  and  the  skylark  of  Europe  have 
been  repeatedly  set  free  in  the  United  States.  But  none  of 
these  colonies  has  long  endured,  perhaps  from  lack  of  adap- 
tation to  the  climate,  more  likely  from  severity  of  competi- 
tion with  other  birds.  In  other  cases  the  introduced  species 
has  been  better  fitted  for  the  conditions  of  life  than  the 
native  forms  themselves,  and  so  has  graduallv  crowded  out 
the  latter.  Both  these  cases  are  illustrated  among  the  rats. 
The  black  rat,  first  introduced  into  America  from  Europe 
about  1544,  helped  crowd  out  the  native  rats,  while  the 
brown  rat,  brought  in  still  later,  about  1775,  in  turn  practi- 
cally exterminated  the  black  rat,  its  fitness  for  the  condir 
tions  of  life  here  being  still  greater  than  that  of  the  other 
European  species. 

Certain  animals  have  followed  man  from  land  to  land, 
having  been  introduced  by  him  against  his  will  and  to  the 
detriment  of  his  domestic  animals  or  crops.  To  many  of 
these  the  term  vermin  has  been  applied.  Among  the  ver- 
min or  "  animal  weeds "  are  certain  of  the  rodents  (rats, 
mice,  rabbits,  etc.),  the  mongoose  of  India,  the  English 
sparrow,  and  many  kinds  of  noxious  insects.  Of  all  the 
vermin  of  this  class  few  have  caused  such  widespread  de- 
struction of  property  as  the  common  European  rabbit  intro- 
duced into  Australia.  The  annual  loss  through  its  presence 
is  estimated  at  $3,500,000. 

It  often  happens  that  man  himself  so  changes  the  en- 
vironment of  a  species  that  it  can  no  longer  maintain  it- 
self. Checking  the  increase  of  a  species,  either  by  actually 
killing  off  its  members  or  by  adverse  change  in  its  sur- 
roundings, is  to  begin  the  process  of  its  destruction.  Cir- 
cumstances become  unfavorable  to  the  growth  or  reproduc- 
tion of  an  animal.  Its  numbers  are  reduced,  fewer  are 
born  each  year,  and  fewer  reach  maturity,  it  grows  rare, 


276  ANIMAL  LIFE 

is  gone,  and  the  final  step  of  extinction  may  often  pass 
unnoticed. 

But  a  few  years  ago  the  air  in  the  Ohio  Valley  was  dark 
in  the  season  of  migration  with  the  hordes  of  passenger 
pigeons.  The  advance  of  a  tree-destroying,  pigeon-shooting 
civilization  has  gone  steadily  on,  and  now  the  bird  which 
once  crowded  our  Western  forests  is  in  the  same  region  an 
ornithological  curiosity.  The  extinction  of  the  American 
bison  or  "  buffalo,"  and  the  growing  rarity  of  the  grizzly 
bear,  the  wolf,  and  of  large  carnivora  generally,  furnishes 
cases  in  point.  When  Bering  and  Steller  landed  on  the 
Commander  Islands  in  1741,  the  sea-cow,  a  large  herbivo- 
rous creature  of  the  shores,  was  abundant  there.  In  about 
fifty  years  the  species,  being  used  for  food  by  fishermen, 
entirely  disappeared.  In  most  cases,  however,  a  species 
that  crosses  its  limiting  barriers,  but  is  unable  to  main- 
tain itself,  leaves  no  record  of  the  occurrence.  We  know,  as 
a  matter  of  fact,  that  stray  individuals  are  very  often  found 
outside  the  usual  limit  of  a  species.  A  tropical  bird  may 
be  found  in  New  Jersey,  a  tropical  fish  on  Cape  Cod,  or  a 
bird  from  Europe  on  the  shores  of  Maine.  Of  course, 
hundreds  of  other  cases  of  this  sort  must  escape  notice ; 
but,  for  one  reason  or  another,  the  great  majority  of  these 
waifs  are  unable  to  gain  a  new  foothold.  For  this  reason, 
outside  of  the  disturbances  created  by  man,  the  geographical 
distribution  of  species  changes  but  little  from  century  to 
century ;  and  yet,  when  we  study  the  facts  more  closely, 
evidences  of  change  appear  everywhere. 

152.  Species  altered  by  adaptation  to  new  conditions. — 
Of  the  third  class  or  species  altered  in  a  new  environment 
examples  are  numerous,  but  in  most  cases  the  causes  in- 
volved can  only  be  inferred  from  their  effects.  One  class 
of  illustrations  may  be  taken  from  island  faunae.  An  island 
is  set  off  from  the  mainland  by  barriers  which  species  of 
land  animals  can  very  rarely  cross.  On  an  island  a  few  waifs 
of  wave  and  storm  may  maintain  themselves,  increasing  in 


FIG.  168— The  manatee,  or  sea-cow  ( Trichechus  latirostris).    A  living  species  of  sea- 
cow  related  to  the  now  extinct  Steller's  sea-cow. 


19 


ANIMAL   LIFE 


FIG.  169.— On  the  shore  of  Narborongh  Island,  one  of  the  Galapagos  Islands,  Pacific 
Ocean,  showing  peculiar  species  of  sea-lions,  lizards,  and  cormorants.  Drawn 
from  a  photograph  made  by  Messrs.  SNODGRASS  and  HELLER. 

numbers  so  as  to  occupy  the  territory ;  but  in  so  doing 
only  those  will  survive  that  can  fit  themselves  to  the  new 
conditions.  Through  this  process  a  new  species  will  be 
formed,  like  the  parent  species  in  general  structure,  but 
having  gained  new  traits  adjusted  to  the  new  environment. 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      279 

The  Galapagos  Islands  are  a  cluster  of  volcanic  rocks 
lying  in  the  open  sea  about  six  hundred  miles  to  the  west 
of  Ecuador.  On  these  islands  is  a  peculiar  land  fauna,  de- 
rived from  South  American  stock,  but  mostly  different  in 
species.  Darwin  noted  there  "twenty-six  land  birds;  of 
these,  twenty-one,  or  perhaps  twenty-three,  are  ranked  as 
distinct  species.  Yet  the  close  affinity  of  most  of  these 
birds  to  American  species  is  manifest  in  every  character,  in 
their  habits,  gestures,  and  tones  of  voice." 

Among  land  animals  similar  migrations  may  occur,  giv- 
ing rise,  through  the  adaptation  to  new  conditions,  to  new 
species.  The  separation  of  species  of  animals  isolated  in 
river  basins  or  lakes  often  permits  the  acquisition  of  new 
characters,  which  is  the  formation  of  distinct  species  in 
similar  fashion.  On  the  west  side  of  Mount  Whitney,  the 
highest  mountain  in  the  Sierra  Xevada  of  California,  there 
is  a  little  stream  called  Volcano  Creek.  In  this  brook  is  a 
distinct  species  or  form  of  trout,  locally  called  golden 
trout.  It  is  unusually  small,  very  brilliantly  colored,  its 
fins  being  bright  golden,  and  its  tiny  scales  scarcely  over- 
lap each  other  along  its  sides.  This  stream  flows  over  a 
high  waterfall  (Agua  Bonita)  into  the  Kern  Eiver.  The 
Kern  Eiver  is. full  of  trout,  of  a  kind  (Salmo  gilberti)  to 
which  the  golden  trout  is  most  closely  allied.  There  can 
not  be  much  doubt  that  the  latter  is  descended  from  the 
former.  With  this  assumption,  it  is  easy  to  suppose  that 
once  the  waterfall  did  not  exist,  or  that  through  some 
agency  we  can  not  now  identify  certain  fishes  had  been 
carried  over  it.  Once  above  it,  they  can  not  now  return, 
nor  can  they  mix  with  the  common  stock  of  the  river. 
Those  best  adapted  to  the  little  stream  have  survived. 
The  process  of  adaptation  has  gone  on  till  at  last  a  distinct 
species  (or  sub-species  *)  is  formed.  In  recent  times  the 

*  In  descriptive  works  the  name  species  is  applied  to  a  form  when 
the  process  of  adaptation  seems  complete.  When  it  is  incomplete,  or 


FIG.  in.— Three  species  of  jack-rabbits,  differing  in  size,  color,  and  markings,  but 
believed  to  be  derived  from  a  common  stock.  The  differences  have  arisen 
through  isolation  and  adaptation.  The  upper  figure  shows  the  head  and  fore  legs 
of  the  black  jack-rabbit  (Lepus  insularis),  of  Espiritu  Santo  Island,  Gulf  of 
California ;  the  lower  right-hand  figure,  the  Arizona  jack-rabbit  (Lepus  alleni), 
specimen  from  Fort  Lowell,  Arizona ;  and  the  lower  left-hand  figure  is  the  San 
Pedro  Martir  jack-rabbit  (Lepus  martirensis),  from  San  Pedro  Martir,  Baja 
California. 


282 


ANIMAL  LIFE 


hand  of  man  has  carried  the  golden  trout  to  other  little 

mountain  torrents,  where  it  thrives  as  well  as  in  the  one 

where  its  peculiarities  were  first  acquired. 

Other  cases  of  this  nature  are  found  among  the  blind 

fishes  of  the  caves  in  different  parts  of  the  world  (Fig.  172). 

In  general,,  caves  are 
formed  by  the  ero- 
sion or  wearing  of 
underground  rivers. 
These  streams  are 
usually  clear  and  cold, 
and  when  they  issue 
to  the  surface  those 
fishes  that  like  cold 
and  shaded  waters 
are  likely  to  enter 
them.  But  to  have 
eyes  in  absolute  dark- 

nCSS,  in  which  no  USC 

-,        ,  ,  , 

can  be  made  °f  them, 
is  a  disadvantage  in 
^  gtmggle  f  Qr  Hf  Q> 

Hence  the  eyed  species  die  or  withdraw,  while  those  in  which 
the  eye  grows  less  from  generation  to  generation,  until  its 
function  is  finally  lost,  are  the  ones  which  survive.  By  such 
processes  the  blind  fishes  in  the  limestone  caves  of  Ken- 
tucky, Indiana,  Tennessee,  and  Missouri  have  been  formed. 


FIG.  172.—  Fishes  showing  stages  in  the  loss  of  eyes 
and  color.  A,  Dismal  Swamp  fish  (  Chologaster 
otXtet),  ancestor  of  the  blind  fish  ;  B,  Agassiz's 
cave  fish  (Chologaster  agassizi);  C,  cave  blind 
fish  (TypWchthys  subterraneu*). 


rather  when  specimens  showing  intergradation  of  characters  are  known, 
the  word  sub-species  is  used.  The  word  variety  has  much  the  same 
meaning  when  used  for  a  subdivision  of  a  species,  but  it  is  a  term 
defined  with  less  exactness.  Thus  the  common  fox  ( Vulpes  pennsyl- 
vanicus)  is  a  distinct  species,  being  separate  from  the  arctic  fox  or  the 
gray  fox  or  the  fox  of  Europe.  The  cross  fox  ( Vulpes  pennsylvanicus 
decussatus)  is  called  a  sub-species,  as  is  the  silver  fox  ( Vulpes  pennsyl- 
vanicus argentatus),  because  these  intergrade  perfectly  with  the  common 
red  fox. 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      283 

To  processes  of  this  kind,  on  a  larger  or  smaller  scale, 
the  variety  in  the  animal  life  of  the  globe  is  very  largely 
due.  Isolation  and  adaptation  give  the  clew  to  the  forma- 
tion of  a  very  large  proportion  of  the  "  new  species "  in 
any  group. 

153.  Effect  of  barriers. — It  will  be  thus  seen  that  geo- 
graphical distribution  is  primarily  dependent  on  barriers  or 
checks  to  the  movement  of  animals.       The  obstacles  met 
in  the  spread  of  animals  determine  the  limits  of  the  spe- 
cies.    Each  species  broadens  its  range  as  far  as  it  can.    It 
attempts  unwittingly,  through  natural  processes  of  increase, 
to  overcome  the  obstacles  of  ocean  or  river,  of  mountain  or 
plain,  of  woodland  or  prairie  or  desert,  of  cold  or  heat,  of 
lack  of  food  or  abundance  of  enemies — whatever  the  bar- 
riers may  be.     Were  it  not  for  these  barriers,  each  type  or 
species  would  become  cosmopolitan  or  universal.     Man  is 
pre-eminently  a  barrier-crossing  animal.    Hence  he  is  found 
in  all  regions  where  human  life  is  possible.     The  different 
races  of  men,  however,  find  checks  and  barriers  entirely 
similar  in  nature  to  those  experienced  by  the  lower  animals, 
and  the  race  peculiarities  are  wholly  similar  to  characters 
acquired  by  new  species  under  adaptation  to  changed  con- 
ditions.    The  degree  of  hindrance  offered  by  any  barrier 
differs  with  the  nature  of  the  species  trying  to  surmount  it. 
That  which  constitutes  an  impassable  obstacle  to  one  form 
may  be  a  great  aid  to  another.     The  river  which  blocks  the 
monkey  or  the  cat  is  the  highway  of  the  fish  or  the  turtle. 
The  waterfall  which  limits  the  ascent  of  the  fish  is  the 
chosen  home  of  the  ouzel.     The  mountain  barrier  which 
the  bobolink  or  the  prairie-dog  does  not  cross  may  be  the 
center  of  distribution  of  the  chief  hare  or  the  arctic  blue- 
bird. 

154.  Relation  of  species  to  habitat. — The  habitat  of  a 
species  of  animal  is  the  region  in  which  it  is  found  in  a 
state  of  Mature.     It  is  currently  believed  that  the  habitat 
of  any  creature  is  the  region  for  which  it  is  best  adapted. 


284  ANIMAL  LIFE 

But  the  reverse  of  this  is  often  true.  There  are  many  cases 
in  which  a  species  introduced  in  a  new  territory,  through 
the  voluntary  or  involuntary  influence  of  man,  has  shown  a 
marvelous  adaptation  and  power  of  persistence.  The  rapid 
spread  of  rahhits  and  pigs  as  wild  animals  in  Australia,  of 
horses  and  cattle  in  South  America,  and  of  the  English 
sparrow  in  North  America,  of  bumble-bees  and  house- 
flies  in  New  Zealand,  are  illustrations  of  this.  Not  one 
of  these  animals  has  maintained  itself  in  the  wild  state 
in  its  native  land  as  successfully  as  in  these  new  countries 
to  which  it  has  been  introduced.  The  work  of  introduc- 
tion of  useful  animals  illustrates  the  same  fact.  The  shad, 
striped  bass,  and  cat-fish  from  the  Potomac  River,  intro- 
duced into  the  Sacramento  River  and  its  tributaries  by  the 
United  States  Fish  Commission,  are  examples  in  point. 
These  valued  food-fishes  are  nowhere  more  at  home  than  in 
the  new  waters  where  no  species  of  their  types  had  ever 
existed  before.  The  carp,  originally  brought  to  Europe 
from  China,  and  thence  to  the  United  States  as  a  food- 
fish,  becomes  in  California  a  nuisance,  which  can  not  be 
eradicated,  destroying  the  eggs  and. the  foodstuff  of  far 
better  fish. 

In  all  mountain  regions  waterfalls  are  likely  to  occur, 
and  these  serve  as  barriers,  preventing  the  ascent  of  trout 
and  other  fishes.  On  this  account  in  the  mountains  of  Cali- 
fornia, Colorado,  Wyoming,  and  other  States,  hundreds  of 
lakes  and  streams  suitable  for  trout  are  found  in  which  no 
fishes  ever  exist.  In  the  Yellowstone  Park  this  fact  is  es- 
pecially noticeable.  This  region  is  a  high  volcanic  plateau, 
formed  by  the  filling  of  an  ancient  granite  basin  with  a  vast 
deposit  of  lava.  The  streams  of  the  park  are  very  cold  and 
clear,  in  every  way  favorable  for  the  growth  of  trout ;  yet, 
with  the  exception  of  a  single  stream,  the  Yellowstone 
River,  none  of  the  streams  was  found  to  contain  any  fish 
in  that  part  of  it  lying  on  the  plateau.  Below  the  plateau 
all  of  them  are  well  stocked.  The  reason  for  this  is  ap- 


286  ANIMAL  LIFE 

parent  in  the  fact  that  the  plateau  is  fringed  with  cataracts 
which  fishes  can  not  ascend.  Each  stream  has  a  canon  or 
deep  gorge  with  a  waterfall  at  its  head,  near  the  point 
where  it  leaves  the  hard  bed  of  black  lava  for  the  rock 
below  (Fig.  173).  So  for  an  area  of  fifteen  hundred  square 
miles  within  the  Yellowstone  National  Park  the  streams 
were  without  trout  because  their  natural  inhabitants  had 
never  been  able  to  reach  them.  When  this  state  of  things 
was  discovered  it  was  easy  to  apply  the  remedy.  Trout  of 
different  species  were  carried  above  the  cascades,  and  these 
have  multiplied  with  great  rapidity. 

The  exception  noted  above,  that  of  the  Yellowstone 
River  itself,  evidently  needs  explanation.  An  abundance 
of  trout  is  found  in  this  river  both  above  and  below  the 
great  falls,  and  no  other  fish  occurs  with  it.  This  anomaly 
of  distribution  is  readily  explained  by  a  study  of  the  tribu- 
taries at  the  head  waters  of  the  river.  When  we  ascend 
above  Yellowstone  Lake  to  the  continental  divide,  we  find 
on  its  very  summit  that  only  about  an  eighth  of  a  mile  of 
wet  meadow  and  marsh,  known  as  Two  Ocean  Pass  (Fig. 
174),  separates  the  drainage  of  the  Yellowstone  from  that 
of  the  Columbia.  A  stream  known  as  Atlantic  Creek  flows 
into  the  Yellowstone,  while  the  waters  of  Pacific  Creek  on 
the  other  side  find  their  way  into  the  Snake  River.  These 
two  creeks  are  connected  by  waterways  in  the  wet  meadow, 
and  trout  may  pass  from  one  to  the  other  without  check. 
Thus  from  the  Snake  River  the  Yellowstone  received  its 
trout,  and  from  the  Yellowstone  they  have  spread  to  the 
streams  tributary  to  the  upper  Missouri. 

This  case  is  a  type  of  the  anomalies  in  distribution  of 
which  the  student  of  zoogeography  will  find  many.  But 
each  effect  depends  upon  some  cause,  and  a  thorough  study 
of  the  surroundings  or  history  of*a  species  will  show  what 
this  cause  may  be.  In  numerous  cases  in  which  fishes  have 
been  found  above  an  insurmountable  cascade,  the  cause  is 
seen  in  a  marsh  flooded  at  high  water,  connecting  one 


288  ANIMAL  LIFE 

drainage  basin  with  another.  An  example  of  this  is  found 
in  Lava  Creek  in  Yellowstone  Park.  Above  Undine1  and 
Wraith  Falls,  both  insurmountable,  are  found  an  abun- 
dance of  trout.  A  marsh  dry  in  summer  connects  Lava 
Creek  with  Black  Tail  Deer  Creek,  a  tributary  of  the 
Yellowstone  and  without  waterfall.  From  the  Yellow- 
stone through  this  creek  and  marsh  the  trout  find  their 
way  into  Lava  Creek.  In  California  numerous  anomalies 
have  been  noted,  as  the  occurrence  of  Tahoe  trout  in 
Feather  Eiver  and  in  the  Blue  Lakes  of  Amador,  which  are 
on  the  other  side  of  the  main  crest  of  the  Sierra  Nevada 
from  Lake  Tahoe,  and  the  occurrence  of  the  Whitney 
golden  trout  in  Lone  Pine  Creek,  another  similar  instance. 
In  each  case  naturalists  have  found  the  man  who  actually 
carried  the  species  across  the  divide.  If  this  matter  had 
been  investigated  a  generation  later,  these  cases  would  have 
been  unexplainable  anomalies  in  geographical  distribution. 
Eeal  causes  are  almost  always  simple  when  they  are  once 
known. 

The  ways  in  which  species  may  cross  barriers  in  a  state 
of  Nature  are  as  varied  as  the  creatures  themselves,  and  far 
more  varied  than  the  actual  barriers.  By  the  long-con- 
tinued process  of  adjustment  to  conditions  with  the  inces- 
sant destruction  of  the  unadapted,  the  various  organisms 
have  become  so  well  fitted  to  their  surroundings  that  the 
casual  observer  may  well  suppose  that  each  inhabits  the 
region  best  fitted  for  it.  Men  have  even  thought  that  the 
conditions  of  life  have  been  fitted  to  the  creatures  them- 
selves, so  perfect  is  this  relation. 

155.  Character  of  barriers  to  distribution. — Taking  the 
animal  kingdom  as  a  who^e,  the  two  great  barriers  modify- 
ing distribution  are  the  presence  of  the  sea  and  changes  in 
temperature.  It  is  only  in  rare  cases  that  any  land  ani- 
mals can  cross  either  of  the  great  oceans,  and  these  rare 
cases  relate  chiefly  to  the  arctic  regions.  For  this  reason 
the  land  faunae  of  Africa,  South  America,  and  Australia 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      289 

have  developed  almost  independently  of  one  another.     To 
the  fresh-water  fishes  the  sea  forms  equally  a  barrier,  and 


even  the  shore-fishes  very  rarely  pass  across  great  depths. 
Relatively  few  of  the  shore-fishes  of  Cuba,  for  example,  ever 
cross  the  deep  Florida  Straits,  and  none  of  those  of  Cali- 
20 


290 


ANIMAL  LIFE 


fornia  ever  reach  Honolulu,  nor  are  Hawaiian  shore-fishes 
ever  seen  on  the  coast  of  California.  For  these  reasons 
natural  boundaries  of  the  great  realms  of  distribution  are 
found  in  the  sea. 

The  other  great  check  to  distribution  is  found  in  heat 
and 'cold.  Most  of  the  tropical  animals  can  not  endure 
frost.  The  arctic  animals,  however  fierce  or  active,  are 
enfeebled  by  heat.  The  timber  line,  north  of  which  and 
above  which  frost  occurs  the  year  round,  therefore  serves 


T""%^9fy;:r 


PIG.  176.— Alligators ;  animals  found  only  in  the  warm  waters  of  tropical  and  sub- 
tropical regions. 

as  a  boundary  of  limitation.  Another  equally  marked  is 
the  frost  line.  Even  the  fishes  of  the  tropics  are  extreme- 
ly sensitive  to  slight  cold.  Off  Florida  Keys  the  cutlass- 
fish  is  sometimes  seen  stiff  and  benumbed  on  the  water, 
where  the  temperature  is  scarcely  below  60°  Fahr.  A 
"  norther "  on  the  Gulf  of  Mexico  will  sometimes  bring 
fishes  which  live  in  considerable  depths  to  the  surface, 
through  chilling  the  water.  These  barriers  are  rarely 
crossed  by  localized  species,  but  many  forms,  especially 
birds,  keep  within  a  relatively  uniform  temperature  through 
migration.  The  summers  are  spent  in  the  north  or  in  the 
mountains,  the  winters  in  districts  that  are  warmer. 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      291 

The  climate,  as  distinct  from  the  temperature,  also 
greatly  influences  many  species.  In  the  Eastern  United 
States  and  in  the  extreme  Northwest,  as  in  Europe  and 
much  of  Asia,  the  atmosphere  is  humid  all  the  year  long. 
Rains  occur  at  intervals  in  the  summer,  and  rain  or  snow  in 
the  winter.  The  green  season  is  from  spring  to  fall,  and  the 
resting  of  plants  is  in  the  winter.  To  this  condition  the 
native  animals  adapt  themselves,  and  this  would  seem  to 
be  the  natural  order  of  things. 

But  as  we  pass  the  Western  plains  of  Nebraska,  Kan- 
sas, and  Texas  this  condition  is  materially  changed.  For 
part  of  the  year  rainfall  is  practically  unknown.  The  air 
becomes  dry,  and  under  the  cloudless  sky  the  greater  part 
of  the  vegetation  ripens  its  seed  and  perishes.  This  is  the 
arid  climate.  When  the  rainfall  is  very  scant  the  region 
is  never  covered  with  verdure,  and  is  known  as  desert. 
Such  great  desert  tracts  are  found  in  parts  of  Wyoming, 
Utah,  Nevada,  Idaho,  Colorado,  Arizona,  New  Mexico,  Cali- 
fornia, as  well  as  in  the  northern  parts  of  Mexico.  In  some 
cases  the  deserts  are  exposed  to  great  heat,  forming  an 
ultra-torrid  region,  as  in  Death  Valley  in  California  and  in 
certain  parts  of  Arizona. 

But  the  arid  region  is  not  as  a  whole  desolate.  In  many 
parts  rain  falls  more  or  less  heavily  for  part  of  the  year, 
bringing  a  rank  growth  of  annual  grasses  and  of  verdure 
in  general.  In  California  this  rainfall  is  in  the  winter,  the 
coldest  part  of  the  year,  and  the  country  is  green  from 
November  or  October  to  June  or  May.  In  Mexico  and 
northward  to  Colorado  the  chief  rainfall  is  in  midsummer, 
the  warmest  part  of  the  year,  and  the  summer  is  the  time 
of  verdure. 

To  all  these  conditions  the  plant  life  must  adapt  itself 
and  with  this  the  animal  life.  But  the  species  that  have 
become  fitted  to  the  arid  habitat  have  undergone  some 
change  in  the  process  and  may  have  become  different  spe- 
cies. It  is,  then,  not  easy  for  them  to  recross  the  barrier 


292  ANIMAL  LIFE 

of  climate  to  compete  with  those  forms  already  adapted. 
For  this  reason  a  marked  change  of  climate  like  a  marked 
change  of  temperature  forms  a  natural  barrier  to  distribu- 
tion and  serves  to  circumscribe  a  natural  fauna. 

Closely  associated  with  climate  is  the  nature  of  forest 
growth,  the  growth  of  grass,  and  in  general  the  development 
of  conditions  which  serve  for  food  or  shelter  to  animals. 
These  conditions  depend  in  part  on  soil,  partly  on  climate 
and  topography,  and  partly  on  the  acts  of  man.  The  for- 
est and  forest  soils,  acting  like  a  great  sponge,  retain  the 
waters  of  the  rainy  season,  and  thus  regulate  the  size  of 
the  streams.  The  stream  that  changes  least  in  volume  is 
most  favorable  to  the  life  of  fishes,  frogs,  and  water  ani- 
mals generally.  The  destruction  of  forests  on  the  moun- 
tain sides  acts  adversely  to  the  life  of  these  creatures  as 
well  as  to  the  interests  of  the  farmer  below  whose  lands 
the  streams  should  water.  When  the  forests  are  destroyed, 
the  great  host  of  wood  creatures,  the  bears,  squirrels,  war- 
blers, various  birds,  beasts,  and  insects  of  the  woods  can  no 
longer  maintain  themselves,  and  grow  rare  and  disappear. 
For  reasons  that  are  obvious  the  conditions  that  produce 
forest,  prairie,  canebrake,  sage -desert,  cactus  -  desert,  and 
the  like  are  potent  in  regulating  the  distribution  of  the 
species.  • 

Still  another  set  of  conditions  depends  on  the  food  sup- 
ply. The  planting  of  orchards  tends  to  multiply  greatly 
the  number  of  individuals  of  those  species  which  prey  upon 
fruit.  When  food  is  abundant  the  severity  of  the  struggle 
for  life  is  relaxed  and  individuals  increase  in  number.  A 
species  may  be  put  to  great  stress  by  the  disappearance  of 
the  animal  or  plant  on  which  it  has  depended.  Each 
change  made  by  man  among  the  wild  animals  or  plants 
may  have  far-reaching  effects  upon  others.  The  coyote  or 
prairie-wolf  destroys  sheep  in  the  ranges  of  the  West.  It 
is  thinned  out  by  means  of  the  bounty  upon  its  scalp. 
Then  the  jack-rabbit,  on  which  it  also  feeds,  greatly  in- 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      293 

creases  in  abundance,  injuring  fruit  trees  and  grain  fields. 
It  is  then  necessary  to  pay  for  its  destruction  also. 

To  destroy  hawks  or  owls  because  they  catch  chickens 
may  increase  the  numbers  and  destructiveness  of  field-mice 
on  which  they  also  prey.  To  shoot  robins,  linnets,  and 
other  birds  that  destroy  small  fruits  is  likely  to  increase 
greatly  the  insect  pests  on  which  these  birds  also  feed. 
The  inter-relations  of  species  and  species  are  so  close  that 
none  should  be  exterminated  by  man  unless  its  habits  and 
relations  have  been  subjected  to  careful  scientific  study. 
Still  less  should  any  new  ones  -be  introduced  without  the 
fullest  consideration  of  the  possible  results.  For  example, 
the  mongoose,  a  weasel-like  creature,  was  introduced  from 
India  into  Jamaica  to  kill  rats  and  mice.  It  killed  also  the 
lizards,  and  thus  produced  a  plague  of  fleas,  an  insect  which 
the  lizards  kept  in  check.  The  English  sparrow,  intro- 
duced that  it  might  feed  on  insects  inhabiting  shade-trees, 
has  become  a  nuisance,  crowding  out  better  birds  and  not 
accomplishing  the  purpose  for  which  it  was  brought  to  the 
United  States. 

To  most  kinds  of  animals  a  mountain  range  must  act  as 
a  barrier  to  distribution.  In  a  region  having  high  moun- 
tains a  species  will  become  in  time  split  up  into  several, 
because  the  individuals  in  one  valley  will  be  isolated  from 
those  of  another.  The  fauna  of  California  furnishes  many 
illustrations  of  this,  as  among  its  mountain  chains  are 
many  deep  valleys  shut  off  from  each  other  and  having 
different  peculiarities  of  temperature.  For  this  reason  two 
counties  of  California  differ  much  more  widely  in  their 
fauna  than  do  two  counties  in  Illinois.  But  Illinois  as  a 
whole  has  more  different  kinds  of  animals  than  California, 
because  no  barrier  anywhere  prevents  their  entrance.  The 
State  has,  we  may  say,  its  doors  wide  open  to  immigrants 
from  all  quarters.  The  same  is  true  of  Iowa  or  of  Kansas 
or  Kentucky.  Illinois  has  a  richer  fauna  than  Iowa,  be- 
cause its  extension  is  north  and  south,  and  it  therefore 
20 


294  ANIMAL  LIFE 

covers  a  wider  range  of  climate.  Kentucky  has  a  richer 
fauna  than  Iowa  because  it  includes  a  greater  variety  of 
conditions.  New  England  was  called  by  Professor  Agassiz 
a  "  zoological  island,"  because  of  the  relatively  small  num- 
ber of  its  native  animals,  especially  of  species  inhabiting  its 
rivers.  The  cause  of  this  is  found  in  its  isolation,  being 
shut  off  from  the  Middle  States  by  mountain  ranges,  while 
it  is  bounded  on  two  sides  by  the  sea. 

156.  Barriers  affecting  fresh-water  animals. — The  animals 
inhabiting  fresh-water  streams  are  affected  by  differences  in 
temperature  and  elevation  much  as  land  animals  are.  They 
tend  to  spread  from  stream  to  stream  whenever  they  can 
find  their  way.  An  isolated  stream  is  likely  to  have  its 
peculiar  fauna  just  as  island  life  is  likely  to  differ  from 
that  of  the  mainland.  The  same  species  wanders  widely 
within  the  limits  of  a  single  river  basin.  If  a  kind  of  fish 
establishes  itself  anywhere  in  the  Mississippi  Valley,  it  may 
find  its  way  to  every  stream  in  the  whole  basin.  If  it  likes 
cold  spring  water,  as  the  rainbow-darter  does,  we  may  look 
for  it  in  any  cold  spring.  If,  like  the  long-eared  sun-fish, 
it  frequents  deep  pools  in  the  brooks,  we  may  look  for  it 
under  roots  of  stumps  and  in  every  "  swimming  hole."  If, 
like  the  channel-cat,  it  chooses  the  ripples  of  a  river,  we 
may  fish  for  it  wherever  ripples  are.  The  larger  the  whole 
river  basin  the  more  species  find  their  way  into  it,  and 
therefore  the  greater  the  number  of  species  in  any  one  of 
its  streams. 

Each  species  finds  its  habitat  fitted  to  its  life,  and  then 
in  turn  is  forced  to  adapt  itself  to  this  habitat.  Any  other 
kind  of  habitat  then  appears  as  a  barrier  to  its  distribu- 
tion. Thus  to  a  fish  of  the  ripples  a  stretch  of  still  water 
becomes  a  barrier.  A  species  adapted  to  sandy  bottoms 
will  seldom  force  its  way  through  swift  waters  or  among 
weeds  or  rocks.  The  effect  of  waterfalls  as  barriers  is  else- 
where noticed.  In  some  streams  the  dam  made  by  a  colony 
of  beavers  has  the  same  effect.  Mill-dams  and  artificial 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      295 

waterfalls  have  checked  the  movements  of  many  species, 
while  others  have  been  helped  by  artificial  channels  or 
canals.  Streams  that  run  muddy  at  times  are  not  favor- 
able for  animal  life.  Still  less  favorable  is  the  condition 
frequent  in  the  arid  region  in  which  streams  are  full  to 
the  banks  in  the  rainy  season  and  shrunk  to  detached 
pools  in  the  dry  months. 

The  stream  that  has  the  greatest  variety  of  animals  in  it 
would  be  one  (1)  connected  with  a  large  river,  (2)  in  a  warm 
climate,  (3)  with  clear  water  and  (4)  little  fluctuation  from 
winter  to  summer,  (5)  with  little  change  in  the  clearness  of 
the  water,  (6)  a  gravelly  bottom,  (7)  preferably  of  lime- 
stone, and  (8)  covered  in  its  quiet  reaches  and  its  ripples 
with  water-weeds.  These  conditions  are  best  realized  in 
tributaries  of  the  Ohio,  Cumberland,  Tennessee,  and  Ozark 
Rivers  among  American  streams,  and  it  is  in  them  that  the 
greatest  number  of  species  of  fresh-water  animals  (fishes, 
cray-fishes,  mussels,  etc.)  has  been  recorded.  These  streams 
approach  most  nearly  to  the  ideal  homes  for  animals  of  the 
fresh  waters.  The  streams  of  Wisconsin,  Michigan,  and  the 
Columbia  region  have  many  advantages,  but  are  too  cold. 
Those  of  Illinois,  Iowa,  northern  Missouri,  and  Kansas  are 
too  sluggish,  and  sometimes  run  muddy.  Those  of  Texas 
and  California  shrink  too  much  in  summer,  and  are  too 
isolated  The  streams  of  the  Atlantic  coast  are  less  iso- 
lated, but  none  connect  with  a  great  basin,  and  those  of 
New  England  run  too  cold  for  the  great  mass  of  the  spe- 
cies. For  similar  reasons  the  fresh-water  animal  life  of 
Europe  is  relatively  scanty,  that  of  the  Danube  and  Volga 
being  richest.  The  animal  life  of  the  fresh  water  of  South 
America  centers  in  the  Amazon,  and  that  of  Africa  in  the 
Nile,  the  Niger,  and  the  Congo.  The  great  rivers  of  Si- 
beria, like  the  Yukon  in  Alaska  and  the  Mackenzie  River 
in  British  America,  have  but  few  forms  of  fresh- water  ani- 
mals, though  those  kinds  fitted  for  life  in  cold,  clear  watei 
exist  in  great  abundance. 


296  ANIMAL   LIFE 

157.  Modes  of  distribution. — The  means  and  modes  of  mi- 
gration and  distribution  are  obvious  in  the  case  of  animals 
that  can  fly  or  swim  or  make  long  journeys  on  foot.     An 
island  can  be  visited  and  become  peopled  by  birds  from  the 
nearest  mainland.     Fishes  and  marine  mammals  can  travel 
from  ocean  to  ocean.     But  many  animals  have  no  means 
of  crossing  watery  barriers.     "  Oceanic  islands,  that  have 
been  formed  de  novo  in  mid-ocean  and  are  not  detached 
portions  of  pre-existing  continents,  are   almost  invariably 
free  from  such  animals  as  are  incapable  of  traversing  the 
sea.     If   sufficiently  distant  from   any  continent,  oceanic 
islands  are  generally  without  mammals,  reptiles,  and  am- 
phibia, but  have  both  birds  and  insects  and  certain  other 
invertebrates  which  are  transported  to  them  by  involuntary 
migration." 

As  suggested  in  the  last  sentence,  migration  may  be 
passive  or  involuntary.  For  example,  those  minute  ani- 
mals that  can  become  dried  up  and  yet  retain  the  power 
of  renewing  their  active  life  under  favorable  conditions  are 
sometimes  carried  in  the  dried  mud  adhering  to  the  feet  of 
birds,  and  may  thus  become  widely  distributed.  Parasites 
are  carried  by  their  hosts  in  all  their  wanderings.  Some 
animals,  as  rats  and  mice,  are  carried  by  ships  and  railway 
trains  and  thus  widely  distributed. 

158.  Fauna  and  faunal  areas. — The  term  fauna  is  applied 
to  the  animals  of  any  region  considered  collectively.     Thus 
the  fauna  of  Illinois  comprises  the  entire  list  of  animals 
found  naturally  in  that  State.     It  includes  the  aboriginal 
men,  the  black  bear,  the  fox,  and  all  its  animal  life  down 
to  the  Amcela.     The  relation  of  the  fauna  of  one  region 
to  that  of  another  depends  on  the  ease  with  which  bar- 
riers may  be  crossed.     Thus  the  fauna  of  Illinois  differs 
little  from  that  of  Indiana  or  Iowa,  because  the  State  con- 
tains no  barriers  that  animals  may  not  readily  pass.     On 
the  other  hand,  the  fauna  of  California  or  Colorado  differs 
materially  from  that  of  adjoining  regions,  because  a  moun- 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      297 

tainous  country  is  full  of  barriers  which,  obstruct  the  diffu- 
sion of  life.  Distinctness  is  in  direct  proportion  to  isola- 
tion. What  is  true  in  this  regard  of  the  fauna  of  any  region 
is  likewise  true  of  its  individual  species.  The  degree  of 
resemblance  among  individuals  is  in  strict  proportion  to  the 
freedom  of  their  movements.  Variation  within  the  limits 
of  a  species  is  again  proportionate  to  the  barriers  which 
prevent  equal  and  free  diffusion. 

159.  Realms  of  animal  life. — The  various  divisions  or 
realms  into  which  the  land  surface  of  the  earth  may  be 
divided  on  the  basis  of  the  character  of  animal  life  have 
their  boundary  in  the  obstacles  offered  to  the  spread  of  the 
average  animal.  In  spite  of  great  inequalities  in  this  regard, 
we  may  yet  roughly  divide  the  land  of  the  globe  into  seven 
principal  realms  or  areas  of  distribution,  each  limited  by 
barriers,  of  which  the  chief  are  the  presence  of  the  sea  and 
the  occurrence  of  frost.  There  are  the  Arctic,  North  Tem- 
perate, South  American,  Indo-African,  Lemurian,  Patago- 
nian,  and  Australian  realms.  Of  these  the  Australian 
realm  alone  is  sharply  denned.  Most  of  the  others  are  sur- 
rounded by  a  broad  fringe  of  debatable  ground  that  forms 
a  transition  to  some  other  zone. 

The  Arctic  realm  includes  all  the  land  area  north  of  the 
isotherm  of  32°.  Its  southern  boundary  corresponds  closely 
with  the  northern  limit  of  trees.  The  fauna  of  this  region 
is  very  homogeneous.  It  is  not  rich  in  species,  most  of  the 
common  types  of  life  of  warmer  regions  being  excluded. 
Among  the  large  animals  are  the  polar  bear,  the  walrus,  and 
certain  species  of  "  ice-riding  "  seals.  There  are  a  few  spe- 
cies of  fishes,  mostly  trout  and  sculpins,  and  a  few  insects. 
Some  of  these,  as  the  mosquito,  are  excessively  numerous 
in  individuals.  Eeptiles  are  absent  from  this  regic/n  and 
many  of  its  birds  migrate  southward  in  the  winter,  finding 
in  the  arctic  only  their  breeding  homes.  When  we  consider 
the  distribution  of  insects  and  other  small  animals  of  wide 
diffusion  we  must  add  to  the  arctic  realm  all  high  moun- 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      299 

tains  of  other  realms  whose  summits  rise  above  the  timber 
line.  The  characteristic  large  animals  of  the  arctic,  as  the 
polar  bear  or  the  musk-ox  or  the  reindeer,  are  not  found 
there,  because  barriers  shut  them  off.  But  the  flora  of  the 
mountain  top,  even  under  the  equator,  may  be  character- 
istically arctic,  and  with  the  flowers  of  the  north  may  be 
found  the  northern  insects  on  whose  presence  the  flower 
depends  for  its  fertilization.  So  far  as  climate  is  concerned 
high  altitude  is  equivalent  to  high  latitude.  On  certain 
mountains  the  different  zones  of  altitude  and  the  corre- 
sponding zones  of  plant  and  insect  life  are  very  sharply 
defined  (Fig.  178). 

The  North  Temperate  realm  comprises  all  the  land  be- 
tween the  northern  limit  of  trees  and  the  southern  limit  of 
frost.  It  includes,  therefore,  nearly  the  whole  of  Europe, 
most  of  Asia,  and  most  of  North  America.  While  there 
are  large  differences  between  the  fauna  of  North  America 
and  that  of  Europe  and  Asia,  these  differences  are  of  minor 
importance  and  are  scarcely  greater  in  any  case  than  the 
difference  between  the  fauna  of  California  and  that  of  our 
Atlantic  coast.  The  close  union  of  Alaska  with  Siberia 
gives  the  arctic  region  an  almost  continuous  land  area  from 
Greenland  to  the  westward  around  to  Norway.  To  the 
south  everywhere  in  the  temperate  zone  realm  the  species 
increase  in  number  and  variety,  and  the  differences  between 
the  fauna  of  North  America  and  that  of  Europe  are  due  in 
part  to  the  northward  extension  into  the  one  and  the  other 
of  types  originating  in  the  tropics.  Especially  is  this  true 
of  certain  of  the  dominant  types  of  singing  birds.  The 
group  of  wood-warblers,  tanagers,  American  orioles,  vireos, 
mocking-birds,  with  the  fly-catchers  and  humming-birds  so 
characteristic  of  our  forests,  are  unrepresented  in  Europe. 
All  of  them  are  apparently  immigrants  from  the  neotropical 
realm  where  nearly  all  of  them  spend  the  winter.  In  the 
same  way  central  Asia  has  many  immigrants  from  the  Indian 
realm  to  the  southward.  With  all  these  variations  there 


GEOGRAPHICAL  DISTRIBUTION  OP  ANIMALS      301 

is  an  essential  unity  of  life  over  this  vast  area,  and  the  rec- 
ognition of  North  America  as  a  separate  (nearctic)  realm, 
which  some  writers  have  attempted,  seems  hardly  practi- 
cable. 

The  Neotropical  or  South  American  realm  includes 
South  America,  the  West  Indies,  the  hot  coast  lands  of 
Mexico,  and  those  parts  of  Florida  and  Texas  where  frost 
does  not  occur.  Its  boundaries  through  Mexico  are  not 
sharply  denned,  and  there  is  much  overlapping  of  the  north 
temperate  realm  along  its  northern  limit.  Its  birds  espe- 
cially range  widely  through  the  United  States  in  the  sum- 
mer migrations,  and  a  large  part  of  them  find  in  the  North 
their  breeding  home.  Southward,  the  broad  barrier  of  the 
two  oceans  keeps  the  South  American  fauna  very  distinct 
from  that  of  Africa  or  Australia.  The  neotropical  fauna  is 
richest  of  all  in  species.  The  great  forests  of  the  Amazon 
are  the  dreams  of  the  naturalists.  Characteristic  types 
among  the  larger  animals  are  the  snout  or  broad-nosed 
(platyrrhine)  monkeys,  which  in  many  ways  are  very  distinct 
from  the  monkeys  and  apes  of  the  Old  World.  In  many  of 
them  the  tip  of  the  tail  is  highly  specialized  and  is  used  as 
a  hand.  The  Edentates  (armadillos,  ant-eaters,  etc.)  are 
characteristically  South  American,  and  there  are  many 
peculiar  types  of  birds,  reptiles,  fishes,  and  insects. 

The  Indo-African  realm  corresponds  to  the  neotropical 
realm  in  position.  It  includes  the  greater  part  of  Africa, 
merging  gradually  northward  into  the  north  temperate 
realm  through  the  transition  districts  which  border  the 
Mediterranean.  It  includes  also  Arabia,  India,  and  the 
neighboring  islands,  all  that  part  of  Asia  south  of  the  limit 
of  frost.  In  monkeys,  carnivora,  ungulates,  and  reptiles 
this  region  is  wonderfully  rich.  In  variety  of  birds,  fishes, 
and  insects  the  neotropical  realm  exceeds  it.  The  monkeys 
of  this  district  are  all  of  the  narrow-nosed  (catarrhine) 
type,  various  forms  being  much  more  nearly  related  to 
man  than  is  the  case  with  the  peculiar  monkeys  of  South 


302 


ANIMAL  LIFE 


America.  Some  of  these  (anthropoid  apes)  have  much 
in  common  with  man,  and  a  primitive  man  derived  from 
these  has  been  imagined  by  Haeckel  and  others.  No 
creature  of  this  character  is  yet  known,  but  that  it  may 
have  once  existed  is  not  impossible.  To  this  region  be- 
long the  eleph'ant,  the  rhinoceros,  and  the  hippopotamus, 
as  well  as  the  lion,  tiger,  leopard,  giraffe,  the  wild  asses, 
and  horses  of  various  species,  besides  a  large  number  of 
ruminant  animals  not  found  in  other  parts  of  the  world. 
It  is,  in  fact,  in  its  lower  mammals  and  reptiles  that  its 

most  striking  dis- 
tinctive characters 
are  found.  In  its 
fish  fauna  it  has 
very  much  in  com- 
mon with  South 
America. 

The  Lemurian 
realm  comprises 
Madagascar  alone. 
It  is  an  isolated  di- 
vision of  the  Indo- 
African  realm,  but 
the  presence  of 
many  species  of 
lemur  and  an  un- 
specialized  or 
primitive  type  of 
lemur  is  held  to 
justify  its  recogni- 
tion as  a  distinct 

realm.  In  most  other  groups  of  animals  the  fauna  of  Mada- 
gascar is  essentially  that  of  neighboring  parts  of  Africa. 

The  Patagonian  realm  includes  the  south  temperate 
zone  of  South  America.  It  has  much  in  common  with  the 
neotropical  realm  from  which  its  fauna  is  mainly  derived, 


FIG.  179.— A  lemur  (Lemur  varius). 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      303 

but  the  presence  of  frost  is  a  barrier  which  vast  numbers  of 
species  can  not  cross.  Beyond  the  Patagonian  realm  lies 
the  Antarctic  continent.  The  scanty  fauna  of  this  region 
is  little  known,  and  it  probably  differs  from  the  Patagonian 
fauna  chiefly  in  the  absence  of  all  but  the  ice-riding  species. 

The  Australian  realm  comprises  Australia  and  the 
neighboring  islands.  It  is  more  isolated  than  any  of  the 
others,  having  been  protected  by  the  sea  from  the  invasions 
of  the  characteristic  animals  of  the  Indo- African  and  tem- 
perate realms.  It  shows  a  singular  persistence  of  low  or 
primitive  types  of  vertebrate  life,  as  though  in  the  process 
of  evolution  the  region  had  been  left  a  whole  geological 
age  behind  the  others.  It  is  certain  that  if  the  closely 
competing  fauna  of  Africa  and  India  could  have  been  able 
to  invade  Australia,  the  dominant  mammals  and  birds  of 
that  region  would  not  have  been  left  as  they  are  now — mar- 
supials and  parrots. 

It  is  only  when  barriers  have  shut  out  competition  that 
simple  or  unspecialized  types  abound.  The  larger  the  land 
area  and  the  more  varied  its  surface,  the  greater  is  the 
stress  of  competition  and  the  more  specialized  are  its  char- 
acteristic forms.  As  part  of  this  specialization  is  in  the 
direction  of  hardiness  and  power  to  persist,  the  species  from 
the  large  areas,  as  a  whole,  are  least  easy  of  extermination. 
The  rapid  multiplication  of  rabbits  and  foxes  in  Australia, 
when  introduced  by  the  hand  of  man,  shows  what  might 
have  taken  place  in  this  country  had  not  impassable  barriers 
of  ocean  shut  them  out. 

160.  Subordinate  realms  or  provinces. — Each  of  these  great 
realms  may  be  indefinitely  subdivided  into  provinces  and 
sections,  for  there  is  no  end  to  the  possibility  of  analy- 
sis. No  school  district  has  exactly  the  same  animals  or 
plants  as  any  other,  as  finally  in  ultimate  analysis  we  find 
that  no  two  animals  or  plants  are  exactly  alike.  Shut  off 
one  pair  of  animals  from  the  others  of  its  species,  and  its 
descendants  will  differ  from  the  parent  stock.  This  differ- 


304  ANIMAL  LIFE 

ence  increases  with  time  and  with  distance  so  long  as  the 
separation  is  maintained.  Hence  new  species  and  new 
fauna  or  aggregations  of  species  are  produced  wherever 
free  diffusion  is  checked  by  any  kind  of  barrier. 

161.  Faunal  areas  of  the  sea. — In  like  manner,  we  may 
divide  the  oceans  into  faunal  areas  or  zones,  according  to 
the  distribution  of  its  animals.  For  this  purpose  the  fishes 
probably  furnish  the  best  indications,  although  results  very 
similar  are  obtained  when  we  consider  the  mollusks  or  the 
Crustacea.  The  fresh-water  fishes  are  not  considered  here, 
as  in  regard  to  their  faunal  areas  they  agree  with  the  land 
animals  of  the  same  regions.  Perhaps  the  most  important 
basis  for  primary  divisions  is  found  in  the  separation  from 
the  localized  shore-fishes  of  the  cosmopolitan  pelagic  species, 
and  the  scarcely  less  widely  distributed  bassalian  species  or 
fishes  of  the  deep  sea. 

The  pelagic  fishes  are  those  which  inhabit  the  open  sea, 
swimming  near  the  surface,  and  often  in  great' schools. 
Such  forms  are  mainly  confined  to  the  warmer  waters. 
They  are  for  the  most  part  predatory  fishes,  strong  swim- 
mers, and  many  of  the  species  are  found  in  all  warm  seas. 
Most  species  have  special  homing  waters,  to  which  they 
repair  in  the  spawning  season.  Often  there  will  be  special 
regions  to  which  they  never  resort,  either  for  breeding  or 
for  food.  At  other  times  a  certain  species  will  appear  in 
numbers  in  regions  where  it  has  hitherto  been  unknown. 
For  example,  the  frigate-mackerel  (Auxis  thazard),  homing 
in  the  East  Indies  and  the  Mediterranean,  appeared  in 
great  numbers  in  1880  off  the  coast  of  New  England.  Typ- 
ical pelagic  fishes  are  the  mackerel,  tunny,  dolphin,  flying- 
fish,  opah,  and  some  species  of  shark.  This  group  shades 
off  by  degrees  into  the  ordinary  shore-fish,  some  being  partly 
pelagic,  venturing  out  for  short  distances,  and  some  are 
pelagic  for  part  of  the  year  only.  To  the  free-swimming 
forms  of  classes  of  animals  lower  than  fishes,  found  in  the 
open  ocean,  the  name  Plankton  is  applied. 


GEOGRAPHICAL  DISTRIBUTION  OF  ANIMALS      3Q5 


The  lassalian  fauna,  or  deep-sea  fauna,  is  composed  of 
species  inhabiting  great  depths  (2,500  feet  to  25,000  feet) 
in  the  sea.  At  a  short  distance  below  the  surface  the 
change  in  temperature  from  day  to  night  is  no  longer  felt. 
At  a  still  lower  depth  there  is  no  difference  between  winter 
and  summer,  and  still  lower  none  between  day  and  night. 
The  bassalian  fishes  in- 
habit a  region  of  great 
cold  and  inky  darkness. 
Their  bodies  are  subjected 
to  great  pressure,  and  the 
conditions  of  life  are  prac- 
tically unvarying.  There 
is  therefore  among  them 
no  migration,  no  seasonal 
change,  no  spawning  sea- 
son fixed  by  outside  con- 
ditions, and  no  need  of 
adaptation  to  varying  en- 
vironment. As  a  result,  all 
are  uniform  indigo-black 
in  color,  and  all  show  more 
or  less  degeneration  in 
those  characters  associated 
with  ordinary  environ- 
ment. Their  bodies  are 
elongate,  from  the  lack  of 
specialization  in  the  ver- 
tebrae. The  flesh,  being 
held  in  place  by  the  great 

pressure  of  the  water,  is  soft  and  fragile.  The  organs  of 
touch  are  often  highly  developed.  The  eye  is  either  exces- 
sively large,  as  if  to  catch  the  slightest  ray  of  light,  or  else 
it  is  undeveloped,  as  if  the  fish  had  abandoned  the  effort 
to  see.  In  many  cases  luminous  spots  or  lanterns  are  de- 
veloped by  which  the  fish  may  see  to  guide  his  way  in  the 
21 


FIG.  180.— A  crinoid  (Rhizocrinus  loxoten- 
sis).  A  deep-sea  animal  which  lives, 
fixed  plant  like,  at  the  bottom  of  the 
ocean. 


306  ANIMAL  LIFE 

sea,  and  in  some  forms  these  shining  appendages  are  highly 
developed.  In  one  form  (^thoprora)  a  luminous  body  cov- 
ers the  end  of  the  nose,  like  the  head-light  of  an  engine. 
In  another  (Ipnops)  the  two  eyes  themselves  are  flattened 
out,  covering  the  whole  top  of  the  head,  and  are  luminous 
in  life.  Many  of  these  species  have  excessively  large  teeth, 
and  some  have  been  known  to  swallow  animals  actually 
larger  than  themselves.  Those  which  have  lantern-like 
spots  have  always  large  eyes. 

The  deep-sea  fishes,  however  fantastic,  have  all  near  rel- 
atives among  the  shore  forms.  Most  of  them  are  degener- 
ate representatives  of  well-known  species — for  example,  of 
eels,  cod,  smelt,  grenadiers,  sculpin,  and  flounders.  The 
deep-sea  crustaceans  and  mollusks  are  similarly  related  to 
shore  forms. 

The  third  great  subdivision  of  marine  animals  is  the 
littoral  or  shore  group,  those  living  in  water  of  moderate 
depth,  never  venturing  far  into  the  open  sea  either  at  the 
surface  or  in  the  depths.  This  group  shades  into  both 
the  preceding.  The  individuals  of  some  of  the  species  are 
excessively  local,  remaining  their  life  long  in  tide  pools  or 
coral  reefs  or  piles  of  rock.  Others  venture  far  from  home, 
and  might  well  be  classed  as  pelagic.  Still  others  ascend 
rivers  either  to  spawn  (anadromous,  as  the  salmon,  shad, 
and  striped  bass),  or  for  purposes  of  feeding,  as  the  robalo, 
corvina,  and  other  shore-fishes  of  the  tropics.  Some  live 
among  rocks  alone,  some  in  sea-weed,  some  on  sandy  shores, 
some  in  the  surf,  and  some  only  in  sheltered  lagoons.  In 
all  seas  there  are  fishes  and  other  marine  animals,  and 
each  creature  haunts  the  places  for  which  it  is  fitted. 


CLASSIFICATION  OF  ANIMALS* 

In  this  diagram  of  classification  every  animal  referred  to  in  this  book,  either  by  its 
vernacular  or  its  scientific  name,  is  assigned  to  its  proper  class  and  branch.  Of  the 
species  mentioned  by  their  scientific  names,  only  the  genus  name  is  given  in  this  list. 


KINGDOM  ANIMALIA 

BRANCH  I.    PEOTOZOA 

CLASS  I.    Rhlzbp'oda. 

Amw'ba,  Globigeri'nse,  Radiola'ria. 
CLASS  II.    Mycetozo'a. 
CLASS  III.    Mastigbph'ora. 

Volvocin'eae,  Go'nium,  Pandorl'na,  Eudorl'na,  Vol'vdx. 
CLASS  IV.    Sporozo  a. 

Grfyari'na. 
CLASS  V.    InfusS  ria. 

PdramoR 'cium,  VorticSl'la. 

BRANCH  II.    PORlF'ERA 

CLASS  I.    Forifera. 

Sponges,    Calcolyrithus,    Prophyse'ma,    Spongtt'la,    Spon'gia, 
Cll'ona. 

BRANCH  III.    CGELfeN'TERA'TA   (se-lgn-te-ra'-ta) 

CLASS  I.    HydrozS'a. 

Hy'dra,  Euco'pe,  Slphonoph'ora,  Physoph'ora,  Obe'lia,  sea-anSm'- 
one,  pol'yp,  Physa'lia,  Pdrapdg' urus. 
CLASS  II.    ScyphozS'a  (sl-fo-zo'-a). 

Jelly-fish,  Lfa'zia. 
CLASS  III.    ActinozS'a. 

C5r'als,  MeM'dium. 
CLASS  IV.    CtSnSph'ora  (ten-5ph'-o-ra). 

*  The  arrangement  of  branches  (or  phyla)  and  classes  here  used  is  that  adopted  in 
Parker  and  HasweH's  Text-Book  of  Zofllogy  (1897). 

307 


308  ANIMAL  LIFE 

BRANCH  IV.    PLATYHELMlN'THES 

CLASS  I.    Turbella'ria. 

Plana'ria. 

CLASS  II.    TrSmatS'da. 
CLASS  III.    Costs' da. 

Tape-worm,  Tce'nia,  Lfy'ula,  flat-worm. 
APPENDIX  TO  PLATYHELMINTHES— CLASS  Nemertin  ea. 


BRANCH  V.    NEMATHELMlN'THES 

CLASS  I.    NSmatS  da. 

Syn'gamus,    round-worm,    Trtchl'na,    Bdthrioceph' alus,    pup« 
worm,  Unclna'ria. 
CLASS  II.    Acanthoceph'ala. 
CLASS  III.    Ohaetbg'natha  (ke-t6g'-na-tha). 

BRANCH  VI.    TROCHELMlN'THES 

CLASS  I.    Rotif'era. 

Rotato'ria. 

CLASS  II.     Dlnophi  lea. 
CLASS  III.    Gastrbt'richa. 

BRANCH  VII.    M6LLU8COI'DA 

CLASS  I.    P51yz6  a. 
CLASS  II.    FhorS'nida. 
CLASS  III.    BrSchibp'oda. 

BRANCH  VIII.    ECHlNODER'MATA 

CLASS  I.    Asteroi  dea. 

Starfish. 

CLASS  II.     Ophiuroi'dea. 
CLASS  III.    Echinoi  dea. 

Sea-urchin. 
CLASS  IV.    HSlothuroi'dea. 

Sea-cucumber. 
CLASS  V.     Crinoi  dea. 

Crinoid,  Rhlzocrl'nus. 
CLASS  VI.    Oystoi'dea. 
CLASS  VII.    Blastoi  dea. 


CLASSIFICATION  OF  ANIMALS  3Q9 


BRANCH  IX.    ANNULA'TA 

CLASS  I.    ChaetSp'oda  (ke-top'-o-da). 

Earth-worm. 

APPENDIX  TO  THE  CH^TOPODA — CLASS  Myzostom Ida, 
CLASS  II.    Gephyre  a  (jef-e-re'-a). 
CLASS  III.    Archi-annel  Ida. 
CLASS  IV.    Hirudin'ea. 

BRANCH  X.    ARTHRtfP'ODA 

CLASS  I.   Crusta'cea. 

Lobster,  cray-fish,  crab,  barnacle,  Le'pas,  hermit-crab,  Pag'urus, 
pea-crab,  Ptnnothe'res,  Epizoari  thus,  fish-lice,  whale-lice,  Saccur 
ll'na,  Lernceo' cera,  prawn,  Pen?  us. 

APPENDIX  TO  CRUSTACEA— CLASS  Trllobl  ta. 

CLASS  II.    OnychSph'ora. 

CLASS  III.    Myriap'oda. 
Cen'tiped. 

CLASS  IV.    InsSc'ta. 

Water- beetle,  water-bug,  canker-worm  moth,  bee,  white  ant, 
cockroach,  mosquito,  weevil,  grasshopper,  caterpillar,  butterfly, 
katydid,  beetle,  Dip'tera,  Lepidop'tera,  monarch  butterfly,  Ano'sia, 
Cu'lex,  Melari oplus,  May-fly,  locust,  cottony-cushion  scale,  Ice'rya, 
lady-bird,  Vedd'lia,  praying-horse,  Man'tis,  Ser'phus,  Cecro'pia, 
gall  insect,  An'dricus,  mole-cricket,  Gryttotal'pa,  Hydroph'ilus, 
Prw'nus,  Campono'tus,  plant-lice,  Aph'idae,  Coc'cidae,  Aphis-lion, 
ant,  Ec'iton.  termite,  bumble-bee,  carpenter-bee,  Andre'na,  HaUc'- 
tus,  yellow-jacket,  hornet,  Ves'pa,  wasp,  At'ta,  bird-lice,  Malloph'- 
aga,  flea,  louse,  Pedic'ulus,  Lipeu'rus,  Hymenop' tera,  ichneumon 
fly,  Thales'sa,  horn-tail,  Tre'mex,  PoUs'tes,  Stylop'idas,  Sty'lops, 
red  orange-scale,  toad-bag,  Gal'gulus,  inch-worm,  span-worm, 
geometrid,  walking-stick,  Diapherom'era,  Phyl'lium,  meadow 
brown,  Grap'ta,  Kal'lima,  sphinx-moth,  tomato-worm,  Phlege- 
thon'tius,  puss-moth,  Ceru'ra,  viceroy  butterfly,  Bdsilar 'chia,  Da- 
na'idae,  Helic5n'idae,  Pier'idas,  PapiliSn'idae,  Syr'phidae,  flower-flies, 
tree-hopper,  Membrac'idae,  Hemip'tera,  Sau'ba  (saw'-ba),  carrion- 
beetle,  Callosa'mia,  prome'thea,  cricket,  cica'da,  dragon-fly,  Cynip'- 
idae,  Hessian-fly,  mud-dauber,  Lere'ma,  Eryn'nis,  skipper  butterfly, 
Schistoc8r'ca. 

CLASS  V.    Arach'nida. 

Tardig'rada,  bear-animalcule,  scorpion,   Lycos'idaB,  tick,    itch- 
mite,  Sarcop'tes,  spider,  trap-door  spider,  turret-spider,  CtenVza. 
21 


310  ANIMAL  LIFE 


BRANCH  XL 

CLASS  I.    FSlecyp'oda. 

Clam,  pond-mussel,  Ll'ma. 
CLASS  II.    Amphineu  ra. 
CLASS  III.    GastrSp  oda. 

Pond-snail,  Lymnas'us,  whelk. 

APPENDIX  TO  THE  GASTROPODA—  CLASS  ScaphSp'oda  AND  Rho'dope. 
CLASS  IV.    CSphalbp'oda. 

BRANCH  XII.    CHORDA'TA 

SUB-BRANCH  I.    AdSlochBr'da.    CLASS  Adelochorda. 

SUB-BRANCH  II.    Urochor'da.    CLASS  Urochorda. 
Sea-squirts,  Tunica'ta. 

SUB-BRANCH  III.    VertebrS'ta.     DIVISION  A.  Acra  nia.    CLASS  Acra* 
nia.    DIVISION  B.  Crania  ta. 

CLASS  I.    OyclostSm'ata. 

CLASS  II.    Pis'ces  (pis'-sez). 

Codfish,  sculpin,  skate,  lady-fish,  Al'bula,  sword-fish,  Xtph'ias, 
flounder,  Platdph'rys,  Salanx,  C.dt'tus,  blob,  miller's  -  thumb, 
conger-eel,  Rgm'ora,  Exocm'tus,  flying-fish,  Cypselu'rus  (sip-se-lu'- 
rus),  deep-sea  angler,  lantern-fish,  Cory  not  ophus,  EcMos'toma, 
JEthoph'ora,  nokee,  scorpion-fish,  IZmmydrfch'thys,  mad-torn, 
jSchilbeodes,  cat-fish,  horned  pout,  toad-fish,  sting-ray,  globe-fish, 
porcupine-fish,  torpedo,  electric  eel,  electric  cat-fish,  star-gazer,  elec- 
tric ray,  Urdl'ophus,  Dl'odon,  Narcl'ne,  Ra'ja,  black-fish,  mud-fish, 
trout,  Sal'  mo,  chub,  horned  dace,  Echeneididae  Amphiprion,  No'- 
meus,  hag-fish,  Myxl'ne,  Septatre'ma,  Polistotre'ma,  lamprey, 
Oligocot'tus,  mouse-fish,  lava-fish,  Pterophryne,  pipe-fish,  Phylldp'- 
teryx,  anglers,  Lo'phius,  Antenna'  rius,  Ceratiide,  minnow,  mack- 
erel, sucker,  salmon,  shad,  alewife,  sturgeon,  striped  bass,  qum'nat, 
eel,  sun-fish,  stickle-back,  carp,  cutlass-fish,  rainbow  darter,  chan- 
nel-cat, Au'xis,  tunny,  dolphin,  opah,  shark,  ^Ethopro'ra,  Ip'nops, 
cod,  smelt,  grenadier,  rob'alo,  corvfna,  Chologas  'ter,  TyphUcti  thys, 
blind-fish. 

CLASS  III.    Amphibia. 

Toad,  frog,  salamander,  tree-frog,  Hi/'la. 

CLASS  IV.    Reptil'ia. 

Tortoise,  snake,  horned  toad,  Phrynoso'ma,  rattlesnake,  lizards, 
An'olis,  chameleon,  Gila  monster,  HelodSr'ma,  Elaps9  coralil'los, 
Lamprop&l'  tia,  Osceola,  alligator. 


CLASSIFICATION  OF  ANIMALS  3H 

CLASS  V.    A'ves. 

Bird-of -paradise,  peacock,  pheasant,  robin,  pigeon,  chicken,  eagle, 
vulture,  guil'lemot  (gil'-e-m5t),  raurre  (mur),  auk,  ful'mar,  pSt'rel, 
sparrow,  bluebird,  woodpecker,  owl,  Colum'ba,  pelican,  Melaner'pes, 
cormorant,  meadow-lark,  warbler,  turkey,  blue  jay,  Aythya,  Cya- 
nocitta,  Uria,  cow-bird,  cuckoo,  parrot,  ptarmigan,  whippoor- 
will,  Antros 'tomus,  gull,  tern,  fly-catcher,  bittern,  mocking-bird, 
shrike,  bobolink,  goose,  humming-bird,  oriole,  puffin,  tailor-bird, 
king-bird,  nightingale,  starling,  skylark,  passenger-pigeon,  ouzel 
(ooz'-el),  linnet,  tanager,  vireo,  wood-warbler,  Phdlacro' corax, 
Trtich'ilus,  Ornithot'omus,  Psaltrtp' arus. 

CLASS  VI.    Mamma  Ha. 

Horse,  ram,  fur-seal,  rabbit,  cat,  ox,  tiger,  lion,  sheep,  elephant, 
whale,  bear,  wolf,  squirrel,  ISm'ming,  fox,  dog,  weasel,  stoat,  rein- 
deer, otter,  ant-eater,  giraffe',  skunk,  porcupine,  hedgehog,  arma- 
dillo, Callorhi'nus,  sea-lion,  deer,  buffalo,  kangaroo,  Mac'ropus, 
duck-bill,  Mon'otreme,  monkey,  gopher,  elk,  bison,  prairie-dog, 
big-horn,  hare,  antelope,  black-tail  deer,  hound,  mole,  hyena,  mice, 
rodent,  woodchuck,  jack-rabbit,  Maca'cus,  C&rcoptth 'ecus,  beaver, 
wood-rat,  pocket-gopher,  coyo'te,  civet-cat,  flying-fox,  mSn'goose, 
sea-cow,  Vul'pes,  walrus,  musk-ox,  ape,  rhmSc'eros,  hippopot'amus, 
leopard,  ass,  le'mur,  TrXch'tchu*,  manatee,  Le'pus,  Odobce'nus, 
Lemur. 


GLOSSARY 


[Only  those  terms  are  defined  in  this  glossary  that  are  not  explained  in  the  text. 
In  the  case  of  the  terms  defined  or  explained  in  the  text,  reference  is  made  to  the 
number  of  the  paragraph  in  which  the  explanation  occurs.  The  pronunciation  of  the 
vernacular  and  scientific  names  of  the  animals  mentioned  in  the  text  is  given  in  the 
Classification.] 

Abomasum:  42. 

Adaptation :  67,  74. 

Albuminous  :  said  of  substances  containing  albumen. 

Alimen  tary  canal :  42. 

Alluring  coloration :  111. 

Altric'ial:  79. 

Altruistic  instinct:  129. 

Amce  bold  :  having  the  changing  form  of  an  Amoeba. 

Anad  romous :  said  of  fishes  that  go  from  the  sea  up  rivers  to  lay  their 
eggs. 

Anatomy:  39. 

Animalcule :  an  animal  of  microscopic  smallness. 

Anten  nae  :  the  "  feelers,"  the  most  anterior  pair  of  appendages  of  in- 
sects and  insect-like  animals  ;  situated  on  the  head,  and  the  seat  of 
organs  of  special  sense. 

An'thropoid:  man-like. 

A'nus :  42. 

Appendix  vermifor  mis  :  82. 

Artificial  selection :  72. 

Assim'ilate :  to  receive  food  and  transform  it  into  a  homogenous  part 
of  the  body  substance. 

Atoll'  :  a  ring-shaped  coral  island  nearly  or  quite  inclosing  a  lagoon. 

Atrophy  :  a  stoppage  of  the  growth  or  development  of  a  part  or  organ. 

Auditory  :  referring  to  the  sense  of  hearing. 

Aut6m  atism :  the  state  of  being  automatic  ;  involuntary  action. 

313 


314;  ANIMAL  LIFE 

Bassalian:  160. 

BIbTogist :  student  of  animals  and  plants. 

Blastoderm:  50. 

Blas'tula:  50. 

Budding :  the  process  of  reproduction  among  animals  in  which  a  small 
part  of  the  body  substance  of  an  animal  grows  out  from  the  sur- 
face, separates  from  the  parent,  and  develops  into  a  new  individual. 

Cae'cum  (se-kum) :  42. 

Carniv'orous :  flesh-eating. 

OSt'arrhine :  nostril  downward ;  said  of  the  narrow-nosed  Old- World 

monkeys. 
Cell :  2. 
Cellulose :  a  peculiar  compound  insoluble  in  all  ordinary  solvents, 

forming  the  fundamental  material  of  the  structure  of  plants,  and 

also  contained  in  the  mantle  of  tunicates. 
OhI'tin:  57. 
ChlS'rophyll :  13. 

Ch.ro  matophore  :  a  aolor-bearing  granule  or  sac. 
Chromosome:  2. 
Chrys'alis :  57. 
Chyle:  42. 
Cilia:  5. 
Cleavage:  50. 
Colon:  42. 
Commen'salism :  90. 
CSmmunal:  83. 
Conjugation:  5. 
Contractile  vacuole :  a  vacuole  that  dilates  and  contracts  regularly, 

and  is  supposed  to  have  an  excretory  function. 
Cyst:  98. 
Cytoplasm :  2. 

Degeneration:  95. 
Development:  46. 
Differentiation :  the  setting  apart  of  special  organs  for  special  work ; 

progressive  change  from  general  to  special ;  specialization. 
Digestion :  the  process  of  dissolving  and  chemically  changing  food  so 

that  it  can  be  assimilated  by  the  blood  and  furnish  nutriment  to 

the  body. 

Dimor'phism :  24. 

Dlvertic'ulum :  a  blind  pouch  arising  from  another  larger  pouch.    44. 
Duode  num:  42. 


GLOSSARY  315 

EC  toblast :  50. 
Ec'toderm:  20. 
Egg-ceU:  20. 
Egois'tic  instinct :  129. 
Embry61'ogy:  39. 
Embrybn  ic  :  49. 
En'dSblast:  50. 
En  do  derm  :  20. 

Environment :  an  organism's  surroundings  taken  collectively. 
Ex'cretory :  referring  to  excretion,  as  excretory  organs,  the  organs 
which  get  rid  of  waste  matter  in  the  animal  body. 

Fau'na  (fawna) :  157. 
Fertilized  egg :  20. 
Fission:  4. 
FlageTla:  13. 
Function:  37. 

Gan  glion  (pi.  ganglia) :  a  nerve-center  composed  of  an  aggregation  of 

nerve-cells. 
Gas'trula:  50. 
Gem  mule:  20. 
Generalization:  41. 

Gebl'ogist :  student  of  the  structure  and  history  of  the  earth. 
Gregarious:  87. 
Growth:  46. 

Habit:  139. 

Herbiv'orous :  plant-eating. 
Heredity:  54. 
Hermaphroditic:  35. 

Hiberna  tion :  passing  the  winter  in  a  death-like  sleep. 

HSmoge  neous :  of  the  same  composition  or  structure  throughout. 

fleum:  42. 

Inorganic :  not  being  nor  having  been  a  living  organism ;  not  organic. 

Insectiv'orous :  insect-eating. 

Instinct:  128. 

Intellect:  139. 

Intercellular :  outside  of  and  between  the  cells. 

Jeju'num:  42. 


316  ANIMAL  LIFE 

Lagoon' :  a  pool  or  lake ;  the  still  water  inclosed  within  an  atoll. 

Lar'va:  57. 

Lepidbp  terous  :  referring  to  the  Lepidoptera,  or  moths  and  butterflies. 

Littoral:  160. 

Lu'men :  the  cavity  of  a  tubular  organ.    44. 

Medusa:  24. 
Meg  alops :  59. 
Mes'oderm:  20. 

Metab  olism:  the  act  or  process  by  which  dead  food  is  built  up  into 
living  matter,  and  living  matter  is  broken  down  into  simpler  prod- 
ucts within  a  cell  or  organism. 

Metamor'phosis :  56. 

Migration:  70. 

Millimeter :  about  one  twenty-fifth  of  an  inch ;  a  term  used  in  the 
metric  system  of  measure. 

Mimicry:  112. 

Mind:  140. 

Molt:  57. 

M6n5g'amous :  said  of  animals  in  which  a  male  mates  with  only  a 
single  female. 

Multiplication:  used  in  the  text  usually  synonymously  with  repro- 
duction. 

Myrmecbph  ilous :  said  of  insects  which  are  found  inhabiting  the  nests 
of  ants. 

Natural  selection:  70. 

No  tochord :  an  elastic  rod,  or  row  of  cells,  formed  in  the  early  embryo 
of  chordate  animals  (including  all  the  vertebrates  and  some  others), 
which  lies  below  the  dorsal  nervous  tube  and  above  the  ventral  ali- 
mentary tube. 

Nucleus  (pi.  nuclei) :  2. 

CEsoph'agus:  42. 

Olfactory :  referring  to  the  sense  of  smell. 

Oma'sum:  42. 

Organ:  37. 

Organic :  referring  to  the  matter  of  which  animals  and  plants  are  corn- 


Organism  :  a  living  being,  plant  or  animal. 
Orientation:  95. 
O'tolith:  121. 


GLOSSARY  317 

Papilla  (pi.  papillae) :  a  small  nipple-like  process,  as  the  papillae  of  the 
skin  or  tongue. 

Parasite:  93. 

Parthenogenesis:  35. 

Pelag'ic :  inhabiting  the  surface  of  mid-ocean.     160. 

Phar'ynx:  42. 

PhysibTogy:  39. 

FlSt'yrrhine :  broad-nosed  ;  said  of  the  New- World  monkeys. 

Plu'teus:  59. 

Polyg'amous  :  said  of  animals  in  which  a  single  male  mates  with  sev- 
eral females :  35. 

Pblymor  phism :  24. 

FbTyp:  21. 

Post-embryonic:  49 

PrsecScial  (pre-co'-shal) :  79. 

Pred  atory :  feeding  on  other  animals. 

Pr5polis:  84. 

Protective  resemblance:  107. 

Protoplasm:  2. 

Pr5ventric'ulus :  42. 

Pseu'dopod:  4. 

Fsy'chic:  140. 

Pupa:  57. 

Reason:  139. 

Recognition  mark :  77,  115. 
Rec'tum:  42. 
Reflex  action :  127. 
Reproduction:  67. 
Respiration:  4. 
Retic'ulum:  42. 
Retina:  123. 
Ru'men :  42. 
Ru'minant :  42. 

Saliva:  42. 

Sensation:  67. 

Sensorium:  126. 

Silica :  the  mineral  of  which  quartz,  sand,  flint,  etc.,  are  composed. 

Spawn,  v. :  to  lay  eggs. 

Specialization:  41. 

Species :  151. 


318  ANIMAL  LIFE 

Sperm  cell :  20. 

Spiracle :  one  of  the  breathing  openings  of  an  insect  situated  on  the 

side  of  the  abdomen  or  thorax. 
Spon'gin:  20. 
Stimulus  (pi.  stimuli) :  that  which  excites  action  in  plant  or  animal 

tissue. 

Stra'ta :  layers,  usually  said  of  rocks. 
Stridula  tion :  122. 
Sub-species :  151. 
Symbiosis:  90. 

Tactile  :  referring  to  the  sense  of  touch.    118. 

Tadpole:  58. 

Ten  tacle  :  a  protruding  flexible  process  or  appendage,  usually  of  the 

head  of  invertebrate  animals,  being  used  as  an  organ  of  touch, 

prehension,  or  motion. 
TermitSph  ilous  :  said  of  insects  inhabiting  the  nests  of  termites. 

VSc'uole :  a  minute  cavity  containing  air,  water,  or  a  chemical  secre- 
tion of  the  protoplasm,  found  in  an  organ,  tissue,  or  cell. 

Vestigial :  82. 

Vis'cera :  the  organs  in  the  great  cavities  of  the  body,  commonly  used 
for  the  organs  in  the  abdominal  cavity. 

Yolk  (yok) :  48. 

Zoe  a :  59. 

Z6oge8g'raphy :  147. 

Zo'oid  :  one  of  the  more  or  less  independent  members  of  a  colonial  or 

compound  organism. 
Zoologist :  a  student  of  animals. 


INDEX 


Abomasum,  68. 

Actinocephalus  oligacanthus  (ill.), 
14. 

Adaptations,  113,  123;  classifica- 
tion of,  123  ;  concerned  with  sur- 
roundings, 143 ;  degree  of  struc- 
tural change  in,  146  ;  for  defense 
of  young,  137 ;  for  rivalry,  135 ; 
for  self-defense,  128  ;  for  secur- 
ing food,  125 ;  origin  of,  123. 

Agassiz's  cave-fish  (illus.),  282. 

Albula  vulpes,  metamorphosis  of 
(illus.),  98. 

Alimentary  canal,  66  ;  of  cock- 
roach (illus.),  73  ;  of  earthworm 
(illus.),  71 ';  of  flatworm  (illus.), 
70;  of  Holothurian  (illus.),  70; 
of  mussel  (illus.),  72  ;  of  Obelia 
(illus.),  69 ;  of  ox  (illus.),  67  ;  of 
Planaria  (illus.),  70 ;  of  sea- 
cucumber  (illus.),  70. 

Alligator  (illus.),  290. 

Alluring  coloration,  216. 

Alternation  of  generations,  42. 

Altricial,  140. 

Amoeba,  5;  multiplication  of  (ill.)  53. 

Amc&ba  polypodia  (illus.),  8. 

Anatomy,  64. 

Andrena,  nest  of  (illus.),  160. 

Andricus  californicus,  galls  of 
(illus.),  143. 


Angler,  deep-sea  (illus.),  124. 

Animals,  life  of  simplest,  1 ;  many- 
celled,  2 ;  one-celled,  2  ;  slightly 
complex,  24. 

Anosia  plexippus,  metamorphosis 
of  (illus.),  92  ;  mimicked  by  Ba- 
silarchia  archippus  (illus.),  219. 

Antenna  of  cray-fish  (illus.),  233  ; 
of  leaf -eating  beetle  (illus.),  230. 

Antennae,  specialized,  of  prome- 
thea  moth  (illus.),  231. 

Antrostomus  vociferus  (illus.),  203. 

Ants  (illus.),  155. 

Anus,  68. 

Appearance,  terrifying,  212. 

Arctic  realm,  297. 

Area,  faunal,  296. 

Artificial  selection,  120. 

Auditory  organ  of  cray-fish  (illus.), 
233;  of  cricket  (illus.),  234;  of 
grasshopper  (illus.),  234 ;  of  mol- 
lusk  (illus.),  233;  of  mosquito 
(illus.),  235. 

Auditory  organs,  232. 

Australian  realm,  303. 

Aythya  (illus.),  137. 

Barbadoes  earth,  19. 

Barnacle,  adult  and  larva  (illus.), 
195;  metamorphosis  of  (illus.), 
101.  319 


320 


ANIMAL  LIFE 


Barrier,  mountains  a,  to  distribu- 
tion, 293 ;  sea  a,  to  distribution, 
288 ;  temperature  a,  to  distribu- 
tion, 290. 

Barriers  affecting  fresh-water  ani- 
mals, 294 ;  effect  of,  283 ;  species 
debarred  by,  274;  to  distribu- 
tion, character  of,  288. 

Basilarchia  archippus  mimicking 
Anosia  plexippus  (illus.),  219. 

Bassalian  fauna,  305. 

Beavers,  nest  of  (illus.),  269. 

Beetle,  larva  of  (illus.),  146. 

Beetles,  lady-bird  (illus.),  214. 

Bird,  egg  of  (illus.),  79. 

Bird-louse  (illus.),  188. 

Bird  of  paradise  (illus.),  58. 

Birds,  nest-making  habits  of,  264. 

Birth,  78. 

Bittern,  nestlings  of  (illus.),  246, 
247. 

Blastoderm,  82. 

Blastula,  82. 

Blue  jay  (illus.),  138. 

Brain,  241. 

Budding,  13. 

Bumble-bee,  159,  (illus.),  161. 

Bush-tit,  nest  of  California  (illus.), 
270. 

Butterfly,  egg  of  (illus.),  79 ;  mon- 
arch, metamorphosis  of  (illus.), 
92. 

Caecum,  68. 

Calcolynthus  porimigenius  (illus.), 
33. 

Calf,  taste  buds  of  (illus.),  229. 

Callorhinus  alascanus  (illus.), 
136. 

Camponotus  (illus.),  155. 

Canadian  skipper  butterfly,  distri- 
bution of  (illus.),  273. 


Canal,  alimentary,  66. 
Cankerworm-moth  (illus.),  59. 
Care  of  young  of  mammals,  268. 
Carpenter-bee,  nest  of  (illus.),  160. 
Caterpillar  parasitized  (illus.),  189, 

(illus.),  190. 

Cave  blind-fish  (illus.),  282. 
Ceanothus  (illus.),  141. 
Cell,  animal,  2 ;  egg,  21,  56 ;  plant, 

2;  products,  3;  wall,  3. 
Cells,  brood,  of  honey-bee  (illus.), 

152;   nerve,  240;   reproductive, 

29,  55. 

Cellulose,  24,  27. 
Centiped  (illus.),  130. 
Cerura,  larva  of  (illus.),  216. 
Chalk,  18. 
Chick,  embryonic  stages  of  (illus.), 

87. 

Chitin,  91. 
Chlorophyll,  24. 

Chologaster  agassizi  (illus.),  282. 
Chologaster  avetus  (illus.),  282. 
Chromatophore,  24. 
Chromosome,  3. 
Chrysalid    of    butterfly,    showing 

protective    resemblance    (illus.), 

206. 

Chrysalis,  93. 
Chyle,  68. 
Cilia,  9. 
Cleavage,  82. 
Climate,  influencing  distribution, 

291 ;  instincts  of,  248. 
Clouded  skipper  butterfly,  distri- 
bution of  (illus.),  273. 
Coccidium  oviforme  (illus.),  14. 
Cockroach,    alimentary    canal    of 

(illus.),  73;  egg  case  of  (illus.), 

140. 
Cocoon  of  Cecropia  moth  (illus.), 

141. 


INDEX 


321 


Colon,  68. 

Colonial  jelly-fishes,  45 ;  Protozoa, 

24 

Colony,  31. 
Color,  222. 

Coloration,  alluring,  216. 
Colors,  warning,  212. 
Commensalism,  172,  173. 
Communal  life,  168;    advantages 

of,  170. 

Communities,  animal,  149. 
Conditions,    primary,    of    animal 

life,  106. 

Conjugating  cells,  28. 
Conjugation,  11,  27,  55. 
Contractile  vacuole,  10. 
Coral,  brain,  45;     island   (illus.), 

44 ;  organ-pipe  (illus.),  45 ;  red, 

45. 

Corals,  37-43. 
Corynolophus   reinhardti    (illus.), 

124. 

Cottony  cushion  scale  (illus.),  142. 
Courtship,  instincts  of,  248. 
Crab,  metamorphosis  of  (illus.),  97 ; 

with  sea-anemone  (illus.),  177. 
Cray-fish,  auditory  organ  of  (illus.), 

233. 
Cricket,  auditory  organ  of  (illus.), 

234. 

Cricket,  mole  (illus.),  146. 
Crinoid  (illus.),  305. 
Crop,  71. 

Crowd  of  animals,  114. 
Crustaceans,    adults    and     larvaB 

(illus.),  195. 
Cteniza  calif ornica,  nest  of  (illus.), 

261. 

Cyanocitta  cristata  (illus.),  138. 
Cycle,  life,  78. 
Cypselurus  (illus.),  131. 
Cytoplasm,  3 
22 


Dead-leaf  butterfly  (illus.),  211. 

Death,  103. 

Deep-sea  angler  (illus.),  124. 

Deer,  horns  of  (illus.),  148. 

Defense  of  the  young,  137. 

Degeneration,  causes  of,  197,  198 ; 
human,  200 ;  through  quiescence, 
193. 

Desiccation,  104. 

Development,  78  :  continuity  of, 
83 ;  divergence  of,  84 ;  embry- 
onic, 80;  first  stages  in  (illus.), 
81 ;  laws  of,  86 ;  metamorphic, 
90;  of  flounder  (illus.),  100;  of 
locust  (illus.),  91 ;  of  vertebrates, 
(illus.),  87;  post-embryonic,  80; 
significance  of  facts  of,  89. 

Diapheromera  femorata  (illus.), 
209. 

Differentiation,  41 ;  of  structure, 
64 

Dimorphism,  42;  sex,  58.,, 

Diodon  hystrix  (illus.),  134. 

Dismal  Swamp  fish  (illus.),  282. 

Distribution,  character  of  barriers, 
to,  288;  geographical,  272;  in- 
fluenced by  climate,  291;  laws 
of,  274;  modes  of,  296;  moun- 
tains a  barrier  to,  293 ;  of  Cana- 
dian Skipper  butterfly  (illus.), 
273 ;  of  clouded  Skipper  butter- 
fly (illus.),  273 ;  of  Erynnis  mani- 
toba  (illus.),  273;  sea  a  barrier 
to,  288;  temperature  a  barrier 
to,  290. 

Diverticula,  74 

Division  of  labor,  22, 168. 

Dog,  pointer  (illus.),  256. 

Dragon-fly,  eye  of  (illus.),  239. 

Duck,  family  (illus.),  137. 

Duodenum,  68. 

Duration  of  life,  101. 


322 


ANIMAL  LIFE 


Earthworm,  alimentary  canal  of 
(illus.),  71. 

Ectoblast,  82. 

Ectoderm,  33. 

Egg  case  of  Californian  barn-door 
skate  (illus.),  140 ;  cockroach  (il- 
lus.), 140. 

Egg  cell,  21,  56. 

Egg,  fertilized,  35 ;  of  bird  (illus.), 
79;  of  butterfly  (illus.),  79;  of 
fish  (illus.),  79;  of  katydid  (il- 
lus.), 79 ;  of  skate  (illus.),  79 ;  of 
toad  (illus.),  79. 

Electric  ray.  (illus.),  135. 

Embryology,  64. 

Embryonic  development,  80;  of 
the  pond  snail,  81. 

Emmydrichthya  vulcanus  (illus.), 
132. 

Endoblast,  82. 

Endoderm,  33. 

Environment,  instincts  of,  248. 

Epizoanthus  paguriphilus,  with 
sea-anemone  (illus.),  177. 

Erynnis  manitoba,  distribution  of 
(illus.),  273. 

Eucope  (illus.),  42. 

Eudorina,  27. 

Eudorina  elegans  (illus.),  28. 

Exocatus  (illus.),  181. 

Eye  of  dragon-fly  (illus.),  239 ;  of 
jelly-fish  (illus.),  238. 

Fauna,  296;  bassalian,  305  ;  littoral, 
306 ;  pelagic,  304. 

Faunal  areas  of  the  sea,  804. 

Feeding  habit  of  Californian  wood- 
pecker (illus.),  128, 129  ;  ofMela- 
nerpes  formicivorus  bairdii 
(illus.),  128,  129;  instincts  of, 
244. 

Female,  57. 


Fish,  egg  of  (illus.),  79 ;  embry- 
onic stages  of  (illus.),  87  ;  -louse 
(illus.),  188. 

Fishes,  man-of-war  (illus.),  175; 
nest-making  habits  of,  264. 

Fission,  9 ;  binary,  54. 

Flagella,  25. 

Flagellata,  24. 

Flatworm,  alimentary  canal  of 
(illus.),  70. 

Flounder,  development  of  (illus.), 
100;  wide-eyed  (illus.),  100. 

Flying  fishes  (illus.),  131. 

Food,  adaptations  for  securing, 
125;  necessary  to  animal  life, 
106. 

Form,  primitive,  20. 

Fossil,  18. 

Fresh-water  animals,  barriers  af- 
fecting, 294. 

Function,  63. 

Fur  seal  (illus.),  136. 

Galapagos    Islands,    animals     of 

(illus.),  278;    locusts  of  (illus.), 

280. 
Gall,  giant,  of  white  oak  (illus.), 

143. 

Galls,  insect,  on  leaf  (illus.),  144. 
Gapes,  worm  which  causes,  60 
Gastrula,  82. 
Gemmule,  35. 
Generalization,  66. 
Generation,  spontaneous,  51. 
Generations,  alternation  of,  51. 
Geographical  distribution,  272. 
Geometrical  larva  on  branch  (illus.), 

209. 
Gerrhonotus    scincicauda    (illus.), 

204. 

Giraffe  (illus.),  126. 
Gizzard,  71. 


INDEX 


323 


Globigerina-ooze,  18. 

GlobigerinaB,  16. 

Gonium,  25-30. 

Oonium  pectorale  (illus.),  25. 

Grasshopper,    auditory    organ   of 

(illus.),  234. 

Green-leaf  insect  (illus.),  210. 
Gregarina,  13,  182. 
Gregarina  polymorpha  (illus.),  14. 
Gregarinidas,  14. 
Gregariousness,  163. 
Growth,  78. 
Gryllotalpa  (illus.),  146. 

Habit,  251. 

Habitat,  relation  of  species  of,  283. 

Habits,  domestic,  257. 

Habits,  nest-making,  of  birds,  264 ; 
of  fishes,  264 ;  of  insects,  262 ;  of 
invertebrates,  258 ;  of  spiders, 
259 ;  of  vertebrates,  264. 

Hearing,  sense  of,  232. 

Reliosphcera  actinota  (illus.),  19. 

Heredity,  89. 

Hermaphroditism,  60. 

Hermit-crab,  with  the  sea-anemone 
(illus.),  176. 

Hibernation,  103. 

Hiving  honey-bees  (illus.),  154. 

Holothurian,  alimentary  canal  of 
(illus.),  70. 

Homes,  257. 

Honey-bee  (illus.),  150 ;  adult  and 
larva  (illus.),  83;  leg  of  (illus.), 
151 :  life  history  of,  149. 

Honey-bees,  hiving  a  swarm  of 
(illus.),  154. 

Host,  relation  of  parasite  to,  179. 

Human  degeneration,  200. 

Humming-bird,  nest  of  rufus 
(illus.),  265,  266. 

Hydra,  37. 


Hydra  vulgans  (illus.),  38. 
Hydrophilus  (illus.),  146. 
Hyla  regilla  (illus.),  145. 

Icerya  and  Vedalia,  121. 

leery  a  purchasi  (illus.),  142. 

Ileum,  68. 

Individual,  31. 

Indo- African  realm,  301. 

Inorganic  matter,  112. 

Insect  galls  on  leaf  (illus.),  144. 

Insects,  metamorphosis  of,  90;  nest- 
making  habits  of,  262 ;  parasitic, 
188. 

Instinct,  242. 

Instincts,  altruistic,  243 ;  classifi- 
cation of,  243 ;  concerned  with 
care  of  the  young,  250 ;  egoistic, 
243;  of  climate,  248;  of  court- 
ship, 248  ;  of  environment,  248 ; 
of  feeding,  244;  of  play,  247;  of 
reproduction,  249:  of  self-de- 
fense, 245  ;  variability  of,  251. 

Intellect,  254. 

Intestine,  68. 

Invertebrates,  nest-making  habits 
of,  258. 

Irritability,  8,  240. 

Island,  coral  (illus.),  44. 

Itch-mite  (illus.),  192. 

Jack-rabbits,    showing    variation 

(illus.),  281. 

Jay,  Canada  (illus.),  138. 
Jejunum,  68. 

Jelly-fish,  eye  of  (illus.),  238. 
Jelly-fishes,  37  ;  colonial,  45. 

Kallima  (illus.),  211. 
Kangaroo  (illus.),  139. 
Katydid,  egg  of  (illus.),  79. 
Lady-bird  beetles  (illus.),  214. 


324: 


ANIMAL  LIFE 


Lady-fish,  metamorphosis  of  (illus.), 
98. 

Larva,  92 ;  of  the  mosquito,  93 ; 
of  butterfly  pupating  (illus.),  94; 
of  the  honey-bee,  152. 

Leaf -cutting  ant  mimicked  by  tree- 
hoppers  (illus.),  220. 

Lemur  (illus.),  302. 

Lemur  varius  (illus.),  302. 

Lemurian  realm,  302. 

Lepas,  adult  and  larva  (illus.),  195  ; 
metamorphosis  of  (illus.),  101. 

Lerema  accius,  distribution  of 
(illus.),  273. 

Lernoeocera  (illus.),  188. 

Life  cycle,  78. 

Life,  communal,  168 ;  duration  of, 
101 ;  primitive,  21 ;  processes, 
21 ;  social,  149. 

Light,  influence  of,  on  animals, 
237. 

Lipeureus  densus  (illus.),  188. 

Littoral  fauna,  306. 

Lizard,  alligator  (illus.),  204. 

Lizzia  koellikeri,  eye  of  (illus.), 
238. 

Locust,  post-embryonic  develop- 
ment of  (illus.),  91. 

Locusts  of  Galapagos  Islands 
(illus.),  280. 

Louse,  sucking  (illus.),  188. 

Macropus  rufus,  139. 

Mad  Tom  (illus.),  132. 

Male,  57. 

Mammals,  care  of  young  of,  268. 

Manatee  (illus.),  277. 

Man-of-war,     Portuguese    (illus.), 

175. 

Mantis  (illus.),  127. 
Many-celled  animal,  2. 
Marine  Protozoa,  15. 


Marks,  recognition,  22,  129,  223. 

Medusae,  41. 

Megalops,  97. 

Melanerpes  formicivorus  bairdii, 
feeding  habit  of  (illus.),  128, 129. 

Membracidae  mimicking  Sauba 
ant  (illus.),  220. 

Mesoderm,  33. 

Metamorphosis,  90  ;  of  Albula  vul- 
pes  (illus.),  98 ;  of  Anosia  plex- 
ippus  (illus.),  92;  of  barnacle 
(illus.),  101 ;  of  butterfly  (illus.), 
92 ;  of  crab  (illus.),  97 ;  of  in* 
sects,  90;  of  lady-fish  (illus.), 
98 ;  of  Lepas  (illus.),  101 ;  of 
mosquito  (illus.),  93 ;  of  sea-ur- 
chin (illus.),  96;  of  sword-fish 
(illus.),  99;  of  toad,  94,  (illus.), 
95 ;  of  Xiphias  gladius  (illus.), 
99. 

Metazoa,  32. 

Metridium  dianthus  (illus.),  43. 

Micro-organism,  16. 

Migration  of  lemming,  118 ;  of  lo- 
cust, 118. 

Mimickry,  218. 

Mind,  255. 

Mining-bee,  nest  of  (illus.),  160. 

Molt,  91. 

Monarch  butterfly  (illus.),  219; 
mimicked  by  Viceroy  butterfly 
(illus.),  219. 

Monogamy,  135. 

Mosquito,  auditor  organ  of  (illus.), 
235  ;  head  of  (illus.),  127 ;  meta- 
morphosis of  (illus.),  93;  young 
stages  of  (illus.),  147. 

Moth,  canker  worm  (illus.),  59. 

Mountains  as  barriers  to  distribu- 
tion, 293. 

Mouth  parts  of  mosquito  (illus.), 
127. 


INDEX 


325 


Mouse-fish  in  gulf-weed  (illus),  208. 
Mt.  Orizaba  (illus.),  300. 
Multiplication,    50 ;    of    animals, 

114;    simplest    method  of,   53; 

slightly  complex  methods  of,  54. 
Murres,  Pallas's  (illus.).  165. 
Mussel,  alimentary  canal  of  (illus.), 

72. 
Mutual  aid,  163. 

Narcine  brasiliensis  (illus.),  135. 

Natural  selection,  117. 

Neotropical  realm,  301. 

Nerve  cells,  240. 

Nerve  fibers,  240. 

Nest-making  habits  of  birds,  264 ; 
of  fishes,  264;  of  insects,  262; 
of  invertebrates,  258 ;  of  spiders, 
259 ;  of  vertebrates,  264. 

Nest  of  Baltimore  oriole  (illus.), 
267;  of  beavers  (illus.),  269;  of 
Californian  bush-tit  (illus.),  270 ; 
of  Cteniza  californica  (illus.), 
261 ;  of  pocket  -  gopher  (illus.), 
271;  of  rufus  humming-bird 
(illus.),  265,  266;  of  tailor-bird 
(illus.),  268 ;  of  trap-door  spider 
(illus.),  261;  of  turret  -  spider 
(illus.),  262. 

Nokee  (illus.),  132. 

Nomeus  gronovii  (illus.),  175. 

North  Temperate  realm,  299. 

Nuclear  membrane,  3. 

Nucleus,  3. 

Obelia,  alimentary  sac  of  (illus.), 

69. 

(Esophagus,  67. 
Omasum,  67. 
One-celled  animals,  2. 
Organ,  63;  auditory,  of  cray-fish 

(illus.),  233;  of  cricket  (illus.), 
22 


234 ;  of  grasshopper  (illus.),  234 ; 

of  mosquito  (illus.),  235 ;  of  mol- 

lusk  (illus.),  233. 
Organic  matter,  112. 
Organs,  auditory,   232;    of  smell, 

229;  of  sound-making,  235;  of 

taste,  228;  of  touch,  226;  spe- 

cialization  of,  66 ;  vestigial,  147. 
Oriole,  Baltimore,,  nest  of  (illus.), 

267. 
Ornithotomous  s^torius,  nest   of, 

268. 

Osprey  Falls  (illus.),  285. 
Otolith,  232. 

Ox,  alimentary  canal  of  (illus.)  67. 
Oxygen,  necessary  to  animal  life, 

107. 

Pagurus  (illus.),  176; 
Pandorina,  26. 
Pandorina  sp.  (illus.),  26. 
Pandorina  morum  (illus.),  27. 
Papilio,  chrysalid  of  (illus.),  205. 
Papilla,  tactile,  of    skin  of  man 

(illus.),  227. 
Papillae,  67. 
Paramcecium,  9. 
ParamoBcium  aurelia  (illus.),  10. 
Paramwcium  caudatum  (illus.),  11. 
ParamcBcium  plutorinum  (illus.), 

11. 
Parasite  of  caterpillar  (illus.),  190; 

relation  to  host,  179. 
Parasites,  simple  structure  of,  181. 
Parasitism,  179;  kinds  of,  180. 
Parthenogenesis,  60. 
Patagonian  realm,  302. 
Pedieulus  (illus.),  188. 
Pelagic  fauna,  304. 
Pelican,  brown  (illus.)j  125. 
Peneus,  adult  and   larva  (illus.), 

195. 


326 


ANIMAL  LIFE 


Pharynx,  71. 

PhlegetJiontius  Carolina,  larva  of 

(illus.),  215. 
Phrynosoma     blainvillei     (illus.), 

131. 

Phyllium  (illus.),  210. 
Phyllopteryx  (illus.),  212. 
Physalia  (illus.),  175. 
Physiology,  64. 
Physoptora  (illus.),  47. 
Pocket-gopher,  nest  of  (illus.),  271. 
Pointer  dog  (illus.)»  256. 
Polistes,    parasitized   by    Stylops 

(illus.),  192. 
Polygamy,  59. 
Polymorphism,  42. 
Polyps,  37. 

Polystomella  strigillata  (illus.),  17. 
Porcupine-fish  (illus.).  134. 
Post-embryonic  development,  80. 
Pigeon  horn-tail  (illus.),  191. 
Pipe-fish  (illus.),  212. 
Planaria,    alimentary    canal    of 

(illus.),  70. 
Plankton,  304. 
Plants,  difference  between  animals 

and,  111. 

Platophrys  lunatus  (illus.),  100. 
Play,  instincts  of,  247. 
Pluteus,  96. 
Praecocial,  140. 

Prawn,  adult  and  larva  (illus.),  195. 
Praying-horse  (illus.),  127. 
Pressure,  a    condition  of  animal 

life,  109. 

Primitive  form,  20. 
Primitive  life,  21. 
Prionus,  larva  of  (illus.),  146. 
Processes,  life,  21. 
Promethea  moth  (illus.),  231. 
Prophysema  primordiale    (illus.), 

34. 


Protective  resemblance,  201. 

Protoplasm,  3;  chemical  constitu- 
tion of,  4 ;  physical  constitution 
of,  4. 

Protozoa,  1 ;  colonial,  24 ;  marine, 
15. 

Psaltriparus  minimus,  nest  of 
(illus.),  270. 

Pseudopod,  5. 

Pterophryne  histrio  in  Sargassum 
(illus.),  208. 

Pupa,  93. 

Puss  moth,  larva  of  (illus.),  216. 

Quiescence,  degeneration  through, 
193. 

Rabbit,  embryonic  stages  of  (illus.), 
87. 

Radiolaria,  16. 

Radiolaria-ooze,  19. 

Raja  binoculata  (illus.),  140. 

Realm,  Arctic,  297;  Australian, 
303;  Indo-African,  301;  Lemu- 
rian,  302;  Neotropical,  301; 
North  Temperate,  299;  of  ani- 
mal life,  297;  Patagonian,  302; 
South  American,  301. 

Realms,  subordinate,  303. 

Reason,  251. 

Recognition  marks,  22, 129, 223. 

Rectum,  68. 

Reflex  action,  241. 

Remora  (illus.),  173. 

Remora  remora  (illus.),  125. 

Reproduction,  9,  50;  instincts  of, 
249. 

Reproductive  cells,  28,  55. 

Resemblance,  aggressive,  202 ;  gen- 
eral protective,  202;  protective, 
201 ;  special  protective,  207 ;  va- 
riable protective,  204. 


INDEX 


327 


Respiration,  7. 

Resting  spore,  28. 

Reticulura,  67. 

Rhizocrinus  loxotenaia  (illus.),  305. 

Rivalry,  adaptations  for,  135. 

Rookeries,  fur-seal  (illus.),  169. 

Rumen,  67. 

Sacculina  (illus.),  187. 

Sacculina,  adult  and  larva  (illus.), 
195. 

Salamander,  embryonic  stages  of 
(illus.),  87. 

Saliva,  67. 

Salmon  leaping  (illus.),  289. 

Salmo  viridens  (illus.),  145. 

Sarcoptes  (illus.),  192. 

Sauba,  ant  mimicked  by  Membrar 
cidcB  (illus.),  220. 

Scale,  red  orange  (illus.),  196. 

Schilbeodes  furiosus  (illus.),  132. 

Schistocerca  (illus.),  280. 

Scorpion  (illus.),  127. 

Scorpion-fish  (illus.),  132. 

Sea,  a  barrier  to  distribution,  288. 

Sea,  faunal  areas  of,  304. 

Sea-anemone,  37;  with  algae  in 
body  (illus.),  178. 

Sea-cow  (illus.),  277. 

Sea-cucumber,  alimentary  canal  of 
(illus.),  70. 

Sea-squirt  (illus.),  194. 

Sea-urchin,  metamorphosis  of  (il- 
lus.), 96. 

Sea-urchins  (illus.),  259. 

Seal,  fur  (illus.),  136;  pups  killed 
by  parasite  (illus.),  186;  rook- 
eries (illus.),  169. 

Selection,artificial,120;natural,117. 

Self-defense,  adaptations  for,  128 ; 
instincts  of,  245. 

Sensation,  8. 


Senses,  special,  224 ;  of  the  simplest 
animals,  225. 

Sensorium,  241. 

Serphus  (illus.),  141. 

Sex,  57 ;  object  of,  57 ;  dimorphism, 
58. 

Shark-clinging  fish  (illus.),  125. 

Sheep,  bighorn  (illus.),  167 ;  Rocky 
Mountain  (illus.),  167. 

Sight,  sense  of,  237. 

Simplest  animals,  life  of,  1. 

Siphonophora,  46. 

Skate,  egg  case  of  California  barn- 
door (illus.),  140;  egg  of  (illus.), 
79. 

Skin  of  man,  tactile    papilla    of    -.- 
(illus.),  227. 

Smell,  sense  of,  229. 

Smelling  organs,  229. 

Smelling  pits  of  leaf-eating  beetle 
(illus.),  230. 

Social  life,  149. 

Sound-making,  235 ;  organs,  235. 

South  American  realm,  301. 

Specialization,  66 ;  of  organs,  66. 

Special  senses,  224. 

Species,  altered  by  adaptation  to 
new  conditions,  276;  debarred 
by  barriers,  274 ;  debarred  by  in- 
ability to  maintain  their  ground, 
275 ;  definition  of,  279 ;  relation 
of,  to  habitat,  283. 

Sperm  cell,  35,  56. 

Sphinx  moth,  larva  of  (illus.),  215. 

Spicules,  sponge,  33. 

Spiders  (illus.),  212;  nest-making 
habits  of,  259. 

Sponges,  32. 

Spongin,  36. 

Spontaneous  generation,  51. 

Spore,  15,  52 ;  resting,  28. 

Sting-ray  (illus.),  133. 


328 


ANIMAL  LIFE 


Structure, 63;  differentiation  of, 64. 

Struggle  for  existence,  116. 

Sty  lops  parasitizing  Polistes  (illus.), 
192. 

Sub-species,  definition  of,  282. 

Surroundings,  adaptations  con- 
cerned with,  143. 

Swallow-tail  butterfly,  chrysalid 
of  (illus.),  205. 

Sword-fish,  metamorphosis  of  (il- 
lus.), 99. 

Symbiosis,  172, 175. 

Syngamus  trachealis  (illus.),  60. 

Systematic  zoology,  64. 

Tactile  organs,  226. 

Tactile  papilla  of  skin  of  man  (il- 
lus.), 227. 

Tadpole,  94. 

Twnia  solium  (illus.),  183. 

Tailor-bird,  nest  of  (illus.),  268. 

Tape-worm  (illus.),  183. 

Taste  buds  of  calf  (illus.),  229. 

Taste  organs,  228. 

Taste,  sense  of,  228. 

Temperature  a  barrier  to  distribu- 
tion, 290 ;  a  condition  of  animal 
life,  108. 

Tentacle,  37. 

Termite,  158,  (illus.),  159. 

Terrifying  appearances,  212. 

Thalessa  lunator  (illus.),  191. 

Toad,  egg  of  (illus.),  79;  horned 
(illus.),  131 ;  metamorphosis  of, 
94,  (illus.),  95. 

Torpedo  (illus.),  135. 

Tortoise,  embryonic  stages  of  (il- 
lus.), 87. 

Touch,  sense  of,  226. 

Trap-door  spider  nest  (illus.),  261. 

Tree-hoppers  mimicking  leaf-cut 
ting  ant  (illus.),  220. 


Tree-toad  (illus.),  145. 

Tremex  columba  (illus.),  191. 

Trichechus  latirostris  (illus.),  277. 

Trichina  spiralis  (illus.),  184. 

Tripoli,  19. 

Trochilus  rufus,  nest   of  (illus.), 

265,  266. 
Trout,  rainbow,  head   of  (illus.), 

145. 

Two  Ocean  Pass  (illus.),  287. 
Tunicate  (illus.),  194. 
Turret-spider,  nest  of  (illus.),  262. 
Typhlichthys  subterraneus  (illus.), 

282. 

Uncinaria,  killing   fur-seal  pups 

(illus.),  186. 

Uria  lomvia  arra  (illus.),  165. 
Urolophus  goodei  (illus.),  133. 

Vacuole,  10 ;  contractile,  10. 
Variety,  definition  of,  282. 
Vedalia  and  Icerya,  121. 
Vertebrates,  early  stages  in  devel- 
opment of  (illus.),  87 ;  nest-mak- 
ing habits  of,  264 ;  parasitic,  193. 
Vespa  (illus.),  162 ;  nest  of  (illus.), 

163. 

Vestigial  organs,  147. 
Viceroy  butterfly  mimicking  Mon- 
arch butterfly  (illus.),  219. 
Voice,  236. 
Volvocinae,  24. 
Volvox,  28. 

Volvox  glolator  (illus.),  29. 
Volvox  minor  (illus.),  29. 
Vorticella,  12. 
Vorticella  microtoma  (illus.),  12. 

Walking-stick  insect  (illus.),  209. 
Warning  colors,  212. 
Walrus,  Atlantic  (illus.),  298. 


INDEX 


329 


Wasps,  social,  161. 

Water -beetle    (illus.),    146;    bug, 

giant  (illus.),  141. 
Whippoorwill  (illus.),  203. 
Woodpecker,  California!!,  feeding 

habit  of  (illus.),  128,  129. 

Xiphias   gladius,  metamorphosis 
of  (illus.),  99. 


Yellow-jacket  (illus.),  162. 

Yolk,  80. 

Young,  adaptations  for  defense  of, 
137;  care  of  the,  250,  257;  num- 
ber of,  61. 

Zoea,  97. 

Zoogeography,  272. 
Zoology,  systematic,  64. 


(13) 


THE   END 


TWENTIETH  CENTURY  TEXT-BOOKS 

A  High  School  Course  in  Physics 

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